Add migration inspection tools for AI agents (bd-627d Phase 2)

Implemented:
- bd migrate --inspect --json: Shows migration plan, db state, warnings
- bd info --schema --json: Returns schema details for agents
- Migration invariants: Validates migrations post-execution
- Added ListMigrations() for introspection

Phase 1 (invariants) and Phase 2 (inspection) complete.
Next: Wire up MCP tools in beads-mcp server.

Amp-Thread-ID: https://ampcode.com/threads/T-c4674660-d640-405f-a929-b664e8699a48
Co-authored-by: Amp <amp@ampcode.com>

internal/storage/sqlite/migrations.go

@@ -29,13 +29,66 @@ var migrations = []Migration{
 	{"content_hash_column", migrateContentHashColumn},
 }
 
-// RunMigrations executes all registered migrations in order
+// MigrationInfo contains metadata about a migration for inspection
+type MigrationInfo struct {
+	Name        string `json:"name"`
+	Description string `json:"description"`
+}
+
+// ListMigrations returns list of all registered migrations with descriptions
+// Note: This returns ALL registered migrations, not just pending ones (all are idempotent)
+func ListMigrations() []MigrationInfo {
+	result := make([]MigrationInfo, len(migrations))
+	for i, m := range migrations {
+		result[i] = MigrationInfo{
+			Name:        m.Name,
+			Description: getMigrationDescription(m.Name),
+		}
+	}
+	return result
+}
+
+// getMigrationDescription returns a human-readable description for a migration
+func getMigrationDescription(name string) string {
+	descriptions := map[string]string{
+		"dirty_issues_table":           "Adds dirty_issues table for auto-export tracking",
+		"external_ref_column":          "Adds external_ref column to issues table",
+		"composite_indexes":            "Adds composite indexes for better query performance",
+		"closed_at_constraint":         "Adds constraint ensuring closed issues have closed_at timestamp",
+		"compaction_columns":           "Adds compaction tracking columns (compacted_at, compacted_at_commit)",
+		"snapshots_table":              "Adds snapshots table for issue history",
+		"compaction_config":            "Adds config entries for compaction",
+		"compacted_at_commit_column":   "Adds compacted_at_commit to snapshots table",
+		"export_hashes_table":          "Adds export_hashes table for idempotent exports",
+		"content_hash_column":          "Adds content_hash column for collision resolution",
+	}
+	
+	if desc, ok := descriptions[name]; ok {
+		return desc
+	}
+	return "Unknown migration"
+}
+
+// RunMigrations executes all registered migrations in order with invariant checking
 func RunMigrations(db *sql.DB) error {
+	// Capture pre-migration snapshot for validation
+	snapshot, err := captureSnapshot(db)
+	if err != nil {
+		return fmt.Errorf("failed to capture pre-migration snapshot: %w", err)
+	}
+
+	// Run migrations (they are already idempotent)
 	for _, migration := range migrations {
 		if err := migration.Func(db); err != nil {
 			return fmt.Errorf("migration %s failed: %w", migration.Name, err)
 		}
 	}
+
+	// Verify invariants after migrations complete
+	if err := verifyInvariants(db, snapshot); err != nil {
+		return fmt.Errorf("post-migration validation failed: %w", err)
+	}
+
 	return nil
 }