docs(agents): simplify and refresh repository guidance

[samzong]

Summary

Simplify the repository agent guide from 98 to 45 lines by consolidating repeated product, architecture, and workflow guidance and linking to the existing contribution and contract documentation. Correct the description of overlays and align authentication guidance with the generated Skill's hostname/source inspection flow.

Verification

• make check
• git diff --cached --check before commit
• Verified that AGENTS.md still links to CLAUDE.md, the staged content matches the working file, all five documentation links resolve, and added lines contain no CJK text.

Compatibility

Documentation only. Preserve the existing agent-guide entry point, architecture boundaries, generated-output restrictions, verification requirements, and release authorization rules. No generated CLI or catalog schema changes.

Checklist

• Tests or focused verification cover the changed surface.
• User-facing behavior changes are documented.
• Generated output under internal/generated/, .cache/, and ad-hoc skills/<cli-name>/ directories is not committed.
• Commits are signed off when this is ready to merge.

[codspeed-hq]

Merging this PR will improve performance by 14.56%

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

⚡ 1 improved benchmark
✅ 20 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	json-large	3.4 ms	3 ms	+14.56%

Tip

Curious why performance improved? Comment @codspeedbot explain why performance improved on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing docs/simplify-agent-guide (ce1ba88) with main (ac2f42c)}