Extract inline hooks to dedicated files and add hook documentation

[PAMulligan]

Summary

7 of 8 hooks are defined as inline bash commands in .claude/settings.json. Only regression-reminder.sh exists as a dedicated file. This makes hooks hard to test, maintain, and understand.

Current State

Hook	Location	Issue
Post-build QA reminder	inline in settings.json	Untestable
Pre-commit token guard	inline in settings.json	Untestable
Dark mode reminder	inline in settings.json	Untestable
Coverage enforcement	inline in settings.json	Untestable
Lighthouse CI	inline in settings.json	Untestable
Bundle size guard	inline in settings.json	Untestable
Mutation testing reminder	inline in settings.json	Untestable
Regression reminder	.claude/hooks/regression-reminder.sh	OK

Additional Gaps

• No user-facing documentation explaining the hook system
• No guide for creating custom hooks
• No error handling in existing hooks
• Hook execution order is undefined

Acceptance Criteria

• Each hook extracted to its own file in .claude/hooks/
• settings.json updated to reference external files
• Each hook file has error handling (exit gracefully on failure)
• Documentation page explaining hook system, execution order, and how to create custom hooks
• Hooks are testable in isolation