feat: improve coding-agent wiki prompts

[Brace Sproul (bracesproul)]

What changed

• Updates generated AGENTS.md and CLAUDE.md guidance to use OpenWiki as optional, just-in-time evidence.
• Refines documentation-generation prompts around ownership, invariants, tests, shipped surfaces, and concise validation.
• Adds prompt and code-mode coverage.

Why

Help coding agents consult repository documentation at high-value decision points while keeping source and tests authoritative.

Validation

• 41 tests
• Typecheck
• Formatting

Replaces #526 with the corrected base branch.

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Latest commit: 6755cbb

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