docs: explain compiler-integrated rules

[kvlonge]

Why

SQLBuild now has one compiler-integrated Rules system, replacing separate lint and project-policy workflows. Users need a coherent path from adoption and configuration through custom authoring and deterministic testing.

Changes

• add a Concepts → Rules group for overview, selection, findings, exceptions, execution, and caching
• add an Advanced Concepts → Custom Rules group for authoring, typed compiler facts, and deterministic testing
• document sqb rules commands, exit behavior, JSON output, and generated agent guidance
• update quickstart, configuration, formatting, contracts, skills, and homepage links

Verification

• validated docs.json
• validated all internal documentation links
• regenerated and tested the packaged SQLBuild documentation skill
• ran the public-repository hygiene scanner