Migrate appdata to ~/.studio/appdata.json with versioned migrations

[bcotrim]

Related issues

Related to STU-1350

How AI was used in this PR

All code was generated via Claude Code (Opus + Haiku). Changes were reviewed and validated through iterative conversation with the user.

Proposed Changes

• CLI _migrate command — Hidden command that copies appdata-v1.json from platform-specific Electron path to ~/.studio/appdata.json
• Desktop bootstrap — Triggers CLI migration on boot before other data operations
• Updated primary paths — Desktop and CLI both read from ~/.studio/appdata.json with CLI fallback to old platform-specific location
• Generic migration framework — New ConfigMigrator in tools/common/lib/ that applies versioned migrations based on file version field
• Wired into config readers — Appdata, CLI config, and shared config all use the migration runner for future extensibility

Testing Instructions

1. npm run typecheck — Verify no type errors
2. npm test — All 1314 tests pass (7 new tests for config migrator)
3. Manual: Launch Desktop with existing appdata-v1.json → Verify ~/.studio/appdata.json is created and Desktop reads from it
4. Manual: Add/modify a site → Verify ~/.studio/appdata.json is updated
5. Manual: CLI studio site list → Verify reads from ~/.studio/appdata.json
6. Manual: Restart Desktop → Verify migration is idempotent (no errors)

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?