Simplify and consolidate settings

[Anton-Horn]

Summary

• remove unused File Explorer and worktree symlink-path settings while retaining fixed internal exclusions
• remove obsolete browser, minimap, and T3 provider controls
• move onboarding and telemetry lifecycle flags into UI state with legacy migration
• consolidate Agent Hooks, flatten unnecessary cards and dividers, and simplify explanatory copy
• improve settings search, dependent states, keyboard accessibility, destructive-action confirmation, and persisted-value validation

Verification

• npm run typecheck
• npm run build
• 75 focused Vitest tests covering settings, migration, runtime behavior, and affected UI
• git diff --check

The production build reports the existing Vite dynamic/static import warnings.