feat(molecules): Add molecules.jsonl as separate catalog file for templates

Implements gt-0ei3: Template molecules now live in a separate molecules.jsonl
file, distinct from work items in issues.jsonl.

Key changes:
- Add internal/molecules package for loading molecule catalogs
- Implement hierarchical loading: built-in → town → user → project
- Molecules use their own ID namespace (mol-*) with prefix validation skipped
- Templates are marked with is_template: true and are read-only
- bd list excludes templates by default (existing functionality)

The hierarchical loading allows:
- Built-in molecules shipped with bd binary (placeholder for future)
- Town-level: ~/gt/.beads/molecules.jsonl (Gas Town)
- User-level: ~/.beads/molecules.jsonl
- Project-level: .beads/molecules.jsonl

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/main.go

@@ -19,6 +19,7 @@ import (
 	"github.com/steveyegge/beads/internal/config"
 	"github.com/steveyegge/beads/internal/debug"
 	"github.com/steveyegge/beads/internal/hooks"
+	"github.com/steveyegge/beads/internal/molecules"
 	"github.com/steveyegge/beads/internal/rpc"
 	"github.com/steveyegge/beads/internal/storage"
 	"github.com/steveyegge/beads/internal/storage/memory"
@@ -633,6 +634,19 @@ var rootCmd = &cobra.Command{
 				autoImportIfNewer()
 			}
 		}
+
+		// Load molecule templates from hierarchical catalog locations (gt-0ei3)
+		// Templates are loaded after auto-import to ensure the database is up-to-date.
+		// Skip for import command to avoid conflicts during import operations.
+		if cmd.Name() != "import" && store != nil {
+			beadsDir := filepath.Dir(dbPath)
+			loader := molecules.NewLoader(store)
+			if result, err := loader.LoadAll(rootCtx, beadsDir); err != nil {
+				debug.Logf("warning: failed to load molecules: %v", err)
+			} else if result.Loaded > 0 {
+				debug.Logf("loaded %d molecules from %v", result.Loaded, result.Sources)
+			}
+		}
 	},
 	PersistentPostRun: func(cmd *cobra.Command, args []string) {
 		// Handle --no-db mode: write memory storage back to JSONL