feat(beads): add built-in molecules for standard workflows

Add three built-in molecule definitions that are automatically seeded
during `gt install`:

- engineer-in-box: Full workflow from design to merge (5 steps)
- quick-fix: Fast path for small changes (3 steps)
- research: Investigation workflow (2 steps)

These molecules provide reusable workflow templates that polecats can
instantiate to execute multi-step procedures with proper dependency
tracking between steps.

Closes gt-4nn.4

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

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

internal/cmd/install.go

@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/spf13/cobra"
+	"github.com/steveyegge/gastown/internal/beads"
 	"github.com/steveyegge/gastown/internal/config"
 	"github.com/steveyegge/gastown/internal/style"
 	"github.com/steveyegge/gastown/internal/templates"
@@ -168,6 +169,13 @@ func runInstall(cmd *cobra.Command, args []string) error {
 			fmt.Printf("   %s Could not initialize town beads: %v\n", style.Dim.Render("⚠"), err)
 		} else {
 			fmt.Printf("   ✓ Initialized .beads/ (town-level beads with gm- prefix)\n")
+
+			// Seed built-in molecules
+			if err := seedBuiltinMolecules(absPath); err != nil {
+				fmt.Printf("   %s Could not seed built-in molecules: %v\n", style.Dim.Render("⚠"), err)
+			} else {
+				fmt.Printf("   ✓ Seeded built-in molecules\n")
+			}
 		}
 	}
 
@@ -240,3 +248,11 @@ func initTownBeads(townPath string) error {
 	}
 	return nil
 }
+
+// seedBuiltinMolecules creates built-in molecule definitions in the beads database.
+// These molecules provide standard workflows like engineer-in-box, quick-fix, and research.
+func seedBuiltinMolecules(townPath string) error {
+	b := beads.New(townPath)
+	_, err := b.SeedBuiltinMolecules()
+	return err
+}