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/git.go

@@ -476,8 +476,8 @@ func CheckSyncBranchConfig(path string) DoctorCheck {
 			Name:    "Sync Branch Config",
 			Status:  StatusWarning,
 			Message: "sync-branch not configured",
-			Detail:  "Multi-clone setups should configure sync-branch in config.yaml",
-			Fix:     "Add 'sync-branch: beads-sync' to .beads/config.yaml",
+			Detail:  "Multi-clone setups should configure sync-branch for safe data synchronization",
+			Fix:     "Run 'bd migrate sync beads-sync' to set up sync branch workflow",
 		}
 	}