docs: add open source contribution guidelines and CI

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -0,0 +1,74 @@
+name: Bug Report
+description: Report a bug in timekeeper-countdown
+title: "[Bug]: "
+labels: ["bug"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for reporting a bug! Please fill out the form below.
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Bug Description
+      description: A clear and concise description of the bug.
+      placeholder: Describe the bug...
+    validations:
+      required: true
+
+  - type: textarea
+    id: reproduction
+    attributes:
+      label: Reproduction Steps
+      description: Steps to reproduce the behavior.
+      placeholder: |
+        1. Create a countdown with...
+        2. Call start()...
+        3. After 5 seconds...
+        4. See error...
+    validations:
+      required: true
+
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected Behavior
+      description: What you expected to happen.
+    validations:
+      required: true
+
+  - type: dropdown
+    id: package
+    attributes:
+      label: Affected Package
+      options:
+        - "@timekeeper-countdown/core"
+        - "@timekeeper-countdown/react"
+        - Both
+    validations:
+      required: true
+
+  - type: textarea
+    id: system-info
+    attributes:
+      label: System Information
+      description: Paste relevant environment details.
+      placeholder: |
+        - Package version:
+        - Node.js version:
+        - OS:
+        - Browser (if applicable):
+      render: text
+
+  - type: checkboxes
+    id: validations
+    attributes:
+      label: Validations
+      options:
+        - label: I have read the [Code of Conduct](https://github.com/eagle-head/timekeeper-countdown/blob/main/CODE_OF_CONDUCT.md)
+          required: true
+        - label: I have searched for [existing issues](https://github.com/eagle-head/timekeeper-countdown/issues)
+          required: true
+        - label: I have tested with the latest version
+          required: true

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,5 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Questions & Help
+    url: https://github.com/eagle-head/timekeeper-countdown/discussions
+    about: Ask questions and get help from the community.

.github/ISSUE_TEMPLATE/feature_request.yml

@@ -0,0 +1,54 @@
+name: Feature Request
+description: Suggest a new feature or improvement
+title: "[Feature]: "
+labels: ["enhancement"]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Thanks for suggesting a feature! Please fill out the form below.
+
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Problem or Motivation
+      description: What problem does this feature solve? Why is it needed?
+      placeholder: I'm always frustrated when...
+    validations:
+      required: true
+
+  - type: textarea
+    id: solution
+    attributes:
+      label: Proposed Solution
+      description: Describe the solution you'd like to see.
+    validations:
+      required: true
+
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives Considered
+      description: Any alternative solutions or workarounds you've considered?
+
+  - type: dropdown
+    id: package
+    attributes:
+      label: Related Package
+      options:
+        - "@timekeeper-countdown/core"
+        - "@timekeeper-countdown/react"
+        - New adapter
+        - Documentation
+    validations:
+      required: true
+
+  - type: checkboxes
+    id: validations
+    attributes:
+      label: Validations
+      options:
+        - label: I have read the [Code of Conduct](https://github.com/eagle-head/timekeeper-countdown/blob/main/CODE_OF_CONDUCT.md)
+          required: true
+        - label: I have searched for [existing issues](https://github.com/eagle-head/timekeeper-countdown/issues)
+          required: true

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,35 @@
+## Description
+
+<!-- What does this PR do? Why is it needed? -->
+
+Fixes #<!-- issue number -->
+
+## Type of Change
+
+<!-- Check the one that applies -->
+
+- [ ] Bug fix (non-breaking change that fixes an issue)
+- [ ] New feature (non-breaking change that adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to change)
+- [ ] Documentation update
+- [ ] Refactoring (no functional changes)
+- [ ] Chore (dependencies, CI, build, etc.)
+
+## How to Test
+
+<!-- Describe how reviewers can test your changes -->
+
+1.
+2.
+3.
+
+## Checklist
+
+- [ ] My code follows the project's code style
+- [ ] I have added/updated tests for my changes
+- [ ] `npm run lint` passes
+- [ ] `npm run typecheck` passes
+- [ ] `npm run test` passes
+- [ ] I have followed the [commit convention](.github/commit-convention.md)
+- [ ] I have added a changeset (`npm run changeset`) if this is a user-facing change
+- [ ] I have updated documentation if needed

.github/commit-convention.md

@@ -0,0 +1,110 @@
+# Commit Convention
+
+This project follows [Conventional Commits](https://www.conventionalcommits.org/).
+
+## Format
+
+```
+<type>(<scope>): <subject>
+
+<body>
+
+<footer>
+```
+
+Only `<type>` and `<subject>` are required. Scope, body, and footer are optional.
+
+## Types
+
+| Type       | Description                          |
+| ---------- | ------------------------------------ |
+| `feat`     | New feature                          |
+| `fix`      | Bug fix                              |
+| `docs`     | Documentation only                   |
+| `style`    | Code style changes (no logic change) |
+| `refactor` | Code refactoring (no feature or fix) |
+| `perf`     | Performance improvement              |
+| `test`     | Adding or updating tests             |
+| `chore`    | Maintenance, dependencies, tooling   |
+| `ci`       | CI/CD configuration                  |
+| `build`    | Build system changes                 |
+
+## Scopes
+
+Scopes are optional. Use them when the change is specific to a package:
+
+| Scope   | When to use                              |
+| ------- | ---------------------------------------- |
+| `core`  | Changes to `@timekeeper-countdown/core`  |
+| `react` | Changes to `@timekeeper-countdown/react` |
+| `docs`  | Changes to the documentation site        |
+| `deps`  | Dependency updates                       |
+
+Omit the scope when the change spans multiple packages or is project-wide.
+
+## Rules
+
+- Use **imperative mood**: "add feature" not "added feature"
+- **Do not capitalize** the first letter of the subject
+- **No period** at the end of the subject
+- Subject line: **max 50 characters**
+- Body: wrap at **72 characters** per line
+- Separate subject from body with a **blank line**
+
+## Breaking Changes
+
+Indicate breaking changes with `!` before the colon:
+
+```
+feat(core)!: remove deprecated startFrom option
+```
+
+Or use the `BREAKING CHANGE:` footer:
+
+```
+feat(core): change snapshot format
+
+BREAKING CHANGE: snapshot.remaining is now in milliseconds instead of seconds
+```
+
+## Examples
+
+```
+feat(react): add onComplete callback to useCountdown
+```
+
+```
+fix(core): correct elapsed time calculation after resume
+```
+
+```
+docs: update getting started guide
+```
+
+```
+test(core): add edge case tests for pause/resume cycle
+```
+
+```
+refactor(core): extract tick logic into separate function
+```
+
+```
+chore(deps): bump vitest to v2
+```
+
+```
+ci: add Node 20 to test matrix
+```
+
+```
+perf(core): reduce unnecessary snapshot allocations
+```
+
+```
+style: apply prettier formatting to config files
+```
+
+```
+build: update tsup config for tree-shaking
+```

.github/workflows/ci.yml

@@ -0,0 +1,65 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  lint:
+    name: Lint & Format
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run lint
+      - run: npm run format:check
+
+  typecheck:
+    name: Type Check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build --workspace @timekeeper-countdown/core
+      - run: npm run typecheck
+
+  test:
+    name: Test (Node ${{ matrix.node-version }})
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        node-version: [18, 20]
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+          cache: npm
+      - run: npm ci
+      - run: npm run test
+
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 20
+          cache: npm
+      - run: npm ci
+      - run: npm run build