[goroyabu]

Summary

• track a concise root AGENTS.md that defines stable f2c repository boundaries
• direct agents to the canonical README, contribution, test, and security guidance instead of duplicating detailed procedures
• remove the root AGENTS.md ignore rule while retaining ignored local documentation notes

Machine-specific interaction rules were moved to the existing global Codex guidance and are intentionally not part of this repository. The superseded ignored documentation-design note was reviewed and removed locally; it is not part of the Git diff.

Related Issue

Closes #21

Verification

• git diff --cached --check completed without errors before the commit
• confirmed every canonical file and template referenced by AGENTS.md exists
• confirmed the global Codex guidance contains the machine-level workflow section exactly once
• confirmed the superseded local documentation-design note was removed
• started a fresh subagent without conversation history and confirmed that it distinguished the global guidance from the f2c-specific guidance, summarized both, and identified the canonical repository documents
• build and CTest were not run because the tracked change affects agent documentation and ignore behavior only

Impact

Fresh clones now receive concise repository-specific agent guidance. Build, runtime, installation, package, CI, and release behavior are unchanged.

Documentation

Adds the repository-owned AGENTS.md. Existing canonical documents remain unchanged.

Checklist

• The change is focused on one coherent maintenance outcome.
• Relevant verification has been run and reported above.
• Canonical guidance remains self-contained and is not duplicated.
• No downloaded archives, generated files, build outputs, or local configuration were committed.