Add orphan detection migration (bd-3852)

Creates migration to detect orphaned child issues and logs them for user
action. Orphaned children are issues with hierarchical IDs (e.g., "parent.child")
where the parent issue no longer exists in the database.

The migration:
- Queries for issues with IDs like '%.%' where parent doesn't exist
- Logs detected orphans with suggested actions (delete, convert, or restore)
- Does NOT automatically delete or convert orphans
- Is idempotent and safe to run multiple times

Test coverage:
- Detects orphaned child issues correctly
- Handles clean databases with no orphans
- Verifies idempotency

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

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

internal/storage/sqlite/migrations.go

@@ -32,6 +32,7 @@ var migrationsList = []Migration{
 	{"repo_mtimes_table", migrations.MigrateRepoMtimesTable},
 	{"child_counters_table", migrations.MigrateChildCountersTable},
 	{"blocked_issues_cache", migrations.MigrateBlockedIssuesCache},
+	{"orphan_detection", migrations.MigrateOrphanDetection},
 }
 
 // MigrationInfo contains metadata about a migration for inspection
@@ -71,6 +72,7 @@ func getMigrationDescription(name string) string {
 		"repo_mtimes_table":            "Adds repo_mtimes table for multi-repo hydration caching",
 		"child_counters_table":         "Adds child_counters table for hierarchical ID generation with ON DELETE CASCADE",
 		"blocked_issues_cache":         "Adds blocked_issues_cache table for GetReadyWork performance optimization (bd-5qim)",
+		"orphan_detection":             "Detects orphaned child issues and logs them for user action (bd-3852)",
 	}
 	
 	if desc, ok := descriptions[name]; ok {