refactor: remove all deletions.jsonl code (bd-fom)

Complete removal of the legacy deletions.jsonl manifest system.
Tombstones are now the sole deletion mechanism.

Removed:
- internal/deletions/ - entire package
- cmd/bd/deleted.go - deleted command
- cmd/bd/doctor/fix/deletions.go - HydrateDeletionsManifest
- Tests for all removed functionality

Cleaned:
- cmd/bd/sync.go - removed sanitize, auto-compact
- cmd/bd/delete.go - removed dual-writes
- cmd/bd/doctor.go - removed checkDeletionsManifest
- internal/importer/importer.go - removed deletions checks
- internal/syncbranch/worktree.go - removed deletions merge
- cmd/bd/integrity.go - updated validation (warn-only on decrease)

Files removed: 12
Lines removed: ~7500

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/autoimport.go

@@ -323,7 +323,6 @@ func importFromGit(ctx context.Context, dbFilePath string, store storage.Storage
 		DryRun:               false,
 		SkipUpdate:           false,
 		SkipPrefixValidation: true,  // Auto-import is lenient about prefixes
-		NoGitHistory:         true,  // Skip git history backfill during auto-import (bd-4pv)
 	}
 
 	_, err = importIssuesCore(ctx, dbFilePath, store, issues, opts)