Restore focused agent workflow documentation

[yiguodev]

Summary

Restore focused guidance for the installed agent skills while keeping API, runtime, build, and verification contracts in AGENTS.md.

AGENTS.md
└── docs/agents/
    ├── issue-tracker.md   GitHub operations and review authorization
    ├── triage-labels.md   Canonical triage label mapping
    └── domain.md         CONTEXT and ADR reading rules

The pull request contains only the documentation changes missing from current main; runtime code and public APIs are unchanged.

Evidence

• Before: GitHub/review guidance was inlined in AGENTS.md, without the focused domain and triage documents.
  After: AGENTS.md links to the three task-specific guides; all 12 local documentation links resolve.
• git diff --check origin/main...HEAD passed. The diff contains exactly four Markdown files.
• go test ./... -count=1 passed on the content-identical source tree. No native artifacts were rebuilt.

Merge Danger

Door: Two-way

Documentation-only; reverting restores the previous layout without changing application data or APIs.

Blast Radius: Documentation

Affects agent instructions for issue handling, reviews, triage, and domain documentation.

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