feat(ci): image arch and simplification

[akijakya]

1. CI to build multi-arch image upon release so it can be used on ARM devices
2. Flatten workflow files into one ci.yaml since neither of the steps were being reused

Fixes #438

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Apr 1, 2026, 1:25 PM

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[Copilot]

Pull request overview

This PR updates the project’s CI/release pipeline to produce a multi-arch (amd64/arm64) OASF server image on versioned releases and simplifies maintenance by flattening previously reusable workflows into a single ci.yaml.

Changes:

• Build and publish ARM64 artifacts for server release tags (v*.*.*) and assemble a multi-arch manifest for the released image.
• Replace reusable GitHub workflow files with a single consolidated .github/workflows/ci.yaml.
• Rename Taskfile build target from build:images to build:server and update docs accordingly.

Reviewed changes

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

Show a summary per file

File	Description
Taskfile.yml	Renames build task to build:server and adds a release:server flow for multi-arch publishing.
README.md	Updates local build instructions to use task build:server.
.github/workflows/ci.yaml	Consolidates build/test/release/helm-release into one workflow and adds multi-arch build steps on release tags.
.github/workflows/reusable-build.yaml	Removed (logic inlined into ci.yaml).
.github/workflows/reusable-test.yaml	Removed (logic inlined into ci.yaml).
.github/workflows/reusable-release.yaml	Removed (logic inlined into ci.yaml).
.github/workflows/reusable-helm-release.yaml	Removed (logic inlined into ci.yaml).

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