Add missing user guides for core framework systems

[PAMulligan]

Summary

Several core framework systems lack user-facing documentation. Users (and contributors) have no way to understand how these systems work beyond reading source code.

Missing Guides

P0 — Core Pipeline Systems

1. Design Token System Guide — token structure, lockfile format, validation, sync strategy, drift detection
2. Visual QA Deep Dive — how visual-diff.js works, sub-pixel detection, typography analysis, region grids, threshold tuning
3. Pipeline Caching & Performance — how incremental builds work, cache invalidation, profiling, when to use --force

P1 — Development Guides

4. Hook System Guide — how hooks fire, execution order, creating custom hooks (pairs with Add JSON schema validation for pipeline.config.json #45)
5. Error Recovery Guide — what to do when a pipeline phase fails, how to resume, common failure modes
6. Agent Creation Guide — how to create a custom agent, required YAML frontmatter, tool declarations

P2 — Framework-Specific

7. Vue Converter Workflow — Vue 3 pipeline specifics, Composition API patterns
8. Svelte Converter Workflow — SvelteKit pipeline specifics, store patterns
9. React Native Converter Workflow — Expo pipeline specifics, NativeWind setup
10. Chrome Extension Pipeline — manifest v3, service worker testing, extension E2E
11. PWA Pipeline — service worker lifecycle, offline testing, manifest validation

Suggested Location

docs/guides/ directory with clear naming (e.g., design-tokens.md, visual-qa.md, caching.md)

Acceptance Criteria

• P0 guides written and linked from CLAUDE.md
• P1 guides written
• P2 guides written (can be shorter, reference existing pipeline docs)
• Onboarding quickstart updated to reference new guides