Remove dead code and stale diagnostics

[FuJacob]

Summary

Remove verified dead code, stale diagnostics, and obsolete tracked artifacts so the shipped app exposes only behavior that has an active caller.

• remove unused suggestion/focus diagnostic mirrors while preserving structured logs and active state-machine behavior
• narrow stale emoji, runtime, settings, permission, onboarding, and API model surfaces; keep assertion-only conveniences in the test target
• delete the obsolete test-target migration script, an unreferenced README image, and a committed Claude lock file; ignore future lock files

Validation

xcodegen generate
# project regenerated with no tracked project-file diff

xcodebuild -project Cotabby.xcodeproj -scheme Cotabby \
  -destination 'platform=macOS' build-for-testing \
  -derivedDataPath build/DerivedData
# ** TEST BUILD SUCCEEDED **

xcodebuild -project Cotabby.xcodeproj -scheme Cotabby \
  -destination 'platform=macOS' test \
  -derivedDataPath build/DerivedData \
  CODE_SIGN_IDENTITY='Apple Development'
# ** TEST SUCCEEDED ** — 1,668 tests, 7 skipped, 0 failures

swiftformat --lint --rules unusedPrivateDeclarations Cotabby CotabbyTests
# 0/404 files require formatting

git diff --check
# clean

Linked issues

None.

Risk / rollout notes

No user-facing feature behavior is intentionally changed. The two broadest removals are the unused FocusInspectionSnapshot construction path and unused @Published suggestion-debug mirrors; focus capability selection, overlay state, correlation IDs, and structured logging remain active and are covered by the full test suite.