feat(doctor): add count-based database size check (#724)

feat(doctor): add count-based database size check

Adds CheckDatabaseSize (Check 29) that warns when closed issues exceed a configurable threshold (default: 5000). The check is informational only - no auto-fix since pruning is destructive.

Also improves fix guidance for sync-branch hook compatibility and legacy JSONL filename checks.

PR #724 by @rsnodgrass

cmd/bd/doctor/git.go

@@ -145,6 +145,8 @@ func CheckSyncBranchHookCompatibility(path string) DoctorCheck {
 			Status:  StatusWarning,
 			Message: "Pre-push hook is not a bd hook",
 			Detail:  "Cannot verify sync-branch compatibility with custom hooks",
+			Fix: "Either run 'bd hooks install --force' to use bd hooks,\n" +
+				"  or ensure your custom hook skips validation when pushing to sync-branch",
 		}
 	}