Add automatic credential sync to workspaces

[gricha]

Summary

• Auto-sync credentials when config is changed via API (no more manual sync button)
• File watcher monitors credential files and syncs on changes with 500ms debounce
• Replace confirmation toast with auto-dismissing "Synced to workspaces" notification
• Document directory sync support in credentials.files

Changes

Agent:

• New FileWatcher class using Bun's native fs.watch()
• Watches credential files, standard locations (~/.gitconfig, ~/.claude, ~/.codex), and SSH keys
• triggerAutoSync() called after all config update handlers

Web UI:

• Simplified SyncToast - auto-dismisses after 2.5s, no buttons
• Renamed useSyncPrompt → useSyncNotification

Test plan

• Integration tests for auto-sync (4 tests)
• API config change triggers sync
• File watcher triggers sync on file change
• Debouncing works for rapid changes
• Directory sync works
• All 105 existing tests pass
• All 20 Playwright e2e tests pass

🤖 Generated with Claude Code