improve: add AGENTS.md with agent instructions and repo context

[Coding-Dev-Tools]

Automated improvement by dev-engineer

[Coding-Dev-Tools]

[Blocked — code conflict] Conflicts with master in src/apiauth/cli.py (application logic), plus .gitattributes, AGENTS.md, pyproject.toml. The cli.py conflict needs a human decision, so I'm not auto-resolving. Please rebase improve/apiauth-1782867471 onto master and resolve cli.py by hand; metadata files can take master's versions.

[github-actions]

🤖 Automated Code Review

✅ Ruff Lint — No issues

⚠️ Ruff Format — Formatting needed

Would reformat: src/apiauth/cli.py
Would reformat: src/apiauth/keygen.py
Would reformat: tests/conftest.py
Would reformat: tests/test_cli.py
4 files would be reformatted, 4 files already formatted

✅ Secret Detection — Clean

✅ Large Files — Within limits

📊 Diff Stats — 2 file(s) changed

 .gitignore | 3 ---
 README.md  | 3 ---
 2 files changed, 6 deletions(-)

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Verdict: ⚠️ Warnings — Lint/format issues found. Recommend fixing before merge.

Automated by Coding-Dev-Tools/.github reusable workflow.