ci: add golangci-lint job and composite action

[ykhdr]

This PR introduces GolangCI-Lint into the CI pipeline.

Changes:

• Added composite action .github/actions/lint now using golangci/golangci-lint-action@v5 for inline PR annotations.
• Updated ci.yml workflow: added lint job with permissions to write PR comments, unit tests depend on lint.
• Added .golangci.yml configuration with a pragmatic baseline of linters.
• Semantics comments added to clarify purpose of changes.

Next steps (optional):

• Extend lint job to upload artifacts only when needed.
• Tune revive and gocritic rules once code style stabilizes.
• Consider enabling fieldalignment and depguard later.

Let me know if you'd like further adjustments.

[Copilot]

Pull Request Overview

This PR introduces GolangCI-Lint to the project and refactors the CI workflow by extracting reusable composite actions for linting and testing.

• Adds comprehensive GolangCI-Lint configuration with multiple linters enabled
• Extracts lint and unit-test workflows into reusable composite GitHub Actions
• Restructures the CI workflow to run linting before tests with proper job dependencies

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
.golangci.yml	New linter configuration defining enabled linters, settings, and exclusion rules
.github/actions/lint/action.yml	New composite action for running GolangCI-Lint with caching
.github/actions/unit-tests/action.yml	New composite action for running Go unit tests with coverage
.github/workflows/ci.yml	Refactored to use composite actions, added lint job, removed matrix strategy

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[ykhdr]

Addressed review feedback:

• Disabled gocyclo and gofumpt in .golangci.yml and removed their settings to avoid confusion (per request).
• Removed redundant actions/checkout from both composite actions; calling workflow performs checkout.
• Honored inputs.working-directory in both actions:
  • lint: passed working-directory to golangci-lint-action and to the Go cache step.
  • unit-tests: set working-directory for the test run and fixed spacing in expressions; also adjusted artifact path.

Notes:

• golangci-lint-action supports version, args, and working-directory inputs; workflow already grants pull-requests: write for annotations.
• If you prefer to keep checkout inside actions (for standalone reuse), we can revert and remove the workflow-level checkout instead.

Let me know if you prefer different linter set or stricter rules, I can tune revive/gocritic accordingly.