docs: deepen rules overview

[kvlonge]

Why

Rules need a first-principles overview that explains the review problem they solve, why SQL linting belongs inside the compiler lifecycle, and where Rules fit alongside compiler correctness, formatting, contracts, tests, and audits.

Changes

• expand the Rules overview with compiler-owned facts and practical review examples
• explain the SQBRSQL subset and the single sqb compile lifecycle
• document built-in and custom Rules, focused runs, findings, exceptions, and cache behavior
• preserve the existing adapter-style overview-plus-detail navigation

Verification

• parsed docs.json
• verified all internal links added to concepts/rules.mdx
• regenerated the packaged SQLBuild skill from this docs tree
• passed public-repository hygiene scan and git diff --check