feat: integrate migration detection into bd doctor (bd-7l27)

Add a consolidated "Pending Migrations" check to bd doctor that:
- Detects sequential ID usage (suggests bd migrate hash-ids)
- Detects legacy deletions.jsonl (suggests bd migrate tombstones)
- Detects missing sync-branch config (suggests bd migrate sync)
- Detects database version mismatches (suggests bd migrate)

Also updates existing checks to use correct modern commands:
- CheckIDFormat: bd migrate hash-ids (was bd migrate --to-hash-ids)
- CheckDeletionsManifest: bd migrate tombstones (was bd migrate-tombstones)
- CheckSyncBranchConfig: bd migrate sync beads-sync (was config.yaml edit)

Removes TODO(bd-7l27) comments from migrate_*.go files.

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

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

cmd/bd/doctor.go

@@ -519,6 +519,11 @@ func runDiagnostics(path string) doctorResult {
 	result.Checks = append(result.Checks, sizeCheck)
 	// Don't fail overall check for size warning, just inform
 
+	// Check 30: Pending migrations (summarizes all available migrations)
+	migrationsCheck := convertDoctorCheck(doctor.CheckPendingMigrations(path))
+	result.Checks = append(result.Checks, migrationsCheck)
+	// Status is determined by the check itself based on migration priorities
+
 	return result
 }