Add shared agent instructions

[revied]

Summary

• Add a canonical root AGENTS.md with repo-specific guidance for Go, Temporal, Helm, Makefile/test/chart workflows, and metrics documentation boundaries.
• Add CLAUDE.md and AGENT.md as symlinks to AGENTS.md so agent-specific entrypoints stay in sync.

Testing

• git diff --cached --check passed before commit.
• Not run: Go tests or chart lint, docs/symlink-only change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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