Add build-packages Copilot skill and route build guidance through build.yml

[Copilot]

This issue requested a dedicated build skill for Copilot that minimizes duplicated instructions and teaches agents to derive build behavior from .github/workflows/build.yml, including scope-based tradeoffs (Node/Python/Java/HTTP). This PR introduces that skill and rewires existing instructions to point to it instead of embedding duplicated build stage details.

• New dedicated build skill

  • Added .github/skills/build-packages.md.
  • Defines .github/workflows/build.yml as the source of truth.
  • Instructs agents to determine required work from target job + needs graph.

• Scope-aware build tradeoffs

  • Documents when to skip out-of-scope rebuild stages for faster iteration:
    • no Node changes → skip Node bundle rebuild
    • no Python changes → skip Python bundle rebuild
    • no Java changes → skip Java agent rebuild
    • no HTTP/C changes → skip cross-arch HTTP library rebuild
  • Clarifies when full workflow-equivalent build is still required for release-quality packaging validation.

• Instruction deduplication and wiring

  • Updated .github/copilot-instructions.md to reference the new skill instead of listing duplicated build-stage command details.
  • Updated .github/constant-instructions.md to route build planning through the new skill and build.yml needs graph.

# .github/skills/build-packages.md (excerpt)

## Source of Truth
- Use `.github/workflows/build.yml` as the canonical build definition.
- Derive what to run from the target job and its `needs` graph.

## Tradeoffs for Faster Task Iteration
- No Node.js changes: skip Node module rebuild work.
- No Python changes: skip Python site-package rebuild work.
- No Java changes: skip Java agent rebuild work.
- No HTTP C injection changes: skip cross-arch HTTP library rebuild work.

[plengauer]

Hello, thanks for contributing for the first time!

[Copilot]

Pull request overview

This PR introduces a dedicated Copilot “build packages” skill and updates existing Copilot instructions to route build guidance through .github/workflows/build.yml (using the job needs graph) instead of duplicating build-stage details in multiple docs.

Changes:

• Added .github/skills/build-packages.md describing how to plan builds from build.yml and when scoped builds may be acceptable.
• Updated .github/copilot-instructions.md to reference the new build skill rather than embedding workflow stage details.
• Updated .github/constant-instructions.md to route build planning through the new skill and build.yml.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
.github/skills/build-packages.md	Adds the new canonical “build packages” skill pointing agents to build.yml and its needs graph.
.github/copilot-instructions.md	Replaces duplicated build-stage documentation with a reference to the new build skill.
.github/constant-instructions.md	Routes build planning through the new build skill and emphasizes scope-minimal builds.