Document each built-in rule in its own folder

[benvinegar]

Summary

• move every built-in rule into its own src/rules/<rule>/ folder with index.ts and a rule-specific README.md
• move shared rule helpers into src/rules/shared/
• link the README's "What it catches" list directly to the matching per-rule docs
• update agent/docs references to reflect the new rule layout

Validation

• bun run format:check
• bun run lint
• bun test
• bun run build

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