feat(sync): add safety check enhancements and merge fixes

- Add forensic logging for mass deletions (bd-lsa): log vanished issue IDs and titles
- Add sync.require_confirmation_on_mass_delete config option (bd-4u8)
- Fix priority merge to treat 0 as "unset" (bd-d0t)
- Fix timestamp tie-breaker to prefer left/local (bd-8nz)
- Add warning log when extraction fails during safety check (bd-feh)
- Refactor safety warnings to return in PullResult (bd-7z4)
- Add TestSafetyCheckMassDeletion integration tests (bd-cnn)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

internal/config/config.go

@@ -99,6 +99,9 @@ func Initialize() error {
 	v.SetDefault("routing.maintainer", ".")
 	v.SetDefault("routing.contributor", "~/.beads-planning")
 
+	// Sync configuration defaults (bd-4u8)
+	v.SetDefault("sync.require_confirmation_on_mass_delete", false)
+
 	// Read config file if it was found
 	if configFileSet {
 		if err := v.ReadInConfig(); err != nil {