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/integrity.go

@@ -103,7 +103,7 @@ func CheckIDFormat(path string) DoctorCheck {
 		Name:    "Issue IDs",
 		Status:  StatusWarning,
 		Message: "sequential (e.g., bd-1, bd-2, ...)",
-		Fix:     "Run 'bd migrate --to-hash-ids' to upgrade (prevents ID collisions in multi-worker scenarios)",
+		Fix:     "Run 'bd migrate hash-ids' to upgrade (prevents ID collisions in multi-worker scenarios)",
 	}
 }
 
@@ -357,7 +357,7 @@ func CheckDeletionsManifest(path string) DoctorCheck {
 					Status:  StatusWarning,
 					Message: fmt.Sprintf("Legacy format (%d entries)", count),
 					Detail:  "deletions.jsonl is deprecated in favor of inline tombstones",
-					Fix:     "Run 'bd migrate-tombstones' to convert to inline tombstones",
+					Fix:     "Run 'bd migrate tombstones' to convert to inline tombstones",
 				}
 			}
 			return DoctorCheck{