chore: streamline agent guidance and focus regression tests

[YishenTu]

Repository agent guides repeated ownership rules and included misleading state/lifetime guidance, while several tests only repeated declarations or asserted the absence of removed code. Consolidate the nine guides around actionable boundaries and verification rules, clarify existing workspace integration, and add test guidance that prioritizes observable behavior.

Remove 10 low-value tests and their unused helpers, trim obsolete menu and renderer source-spelling assertions, and check appearance propagation against declared theme colors. Preserve synchronization, recovery, security, and native lifecycle coverage. No production code changes.

Validation passed:

• 31 focused unit tests and 5 close/lifecycle E2E tests on arm64; the runs built the affected Debug targets.
• Swift lint, architecture guard, architecture fixtures, test-suite boundary checks, exact CLAUDE.md companion checks, and git diff whitespace checks.

Full Release/performance verification was not run for this instruction and test-only cleanup.