feat(beads): add mol-install-go-binary molecule (gt-g44u.2)

Add a single-step molecule for building and installing the gt binary.

The molecule contains one step:
- install: Build with `go build -o gt ./cmd/gt` and install with
  `go install ./cmd/gt`

Added InstallGoBinaryMolecule() function and test.

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

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

internal/beads/builtin_molecules.go

@@ -14,6 +14,7 @@ func BuiltinMolecules() []BuiltinMolecule {
 		EngineerInBoxMolecule(),
 		QuickFixMolecule(),
 		ResearchMolecule(),
+		InstallGoBinaryMolecule(),
 	}
 }
 
@@ -97,6 +98,31 @@ Needs: investigate`,
 	}
 }
 
+// InstallGoBinaryMolecule returns the install-go-binary molecule definition.
+// This is a single step to rebuild and install the gt binary after code changes.
+func InstallGoBinaryMolecule() BuiltinMolecule {
+	return BuiltinMolecule{
+		ID:    "mol-install-go-binary",
+		Title: "Install Go Binary",
+		Description: `Single step to rebuild and install the gt binary after code changes.
+
+## Step: install
+Build and install the gt binary locally.
+
+Run from the rig directory:
+` + "```" + `
+go build -o gt ./cmd/gt
+go install ./cmd/gt
+` + "```" + `
+
+Verify the installed binary is updated:
+` + "```" + `
+which gt
+gt --version  # if version command exists
+` + "```",
+	}
+}
+
 // SeedBuiltinMolecules creates all built-in molecules in the beads database.
 // It skips molecules that already exist (by title match).
 // Returns the number of molecules created.

internal/beads/builtin_molecules_test.go

@@ -5,8 +5,8 @@ import "testing"
 func TestBuiltinMolecules(t *testing.T) {
 	molecules := BuiltinMolecules()
 
-	if len(molecules) != 3 {
-		t.Errorf("expected 3 built-in molecules, got %d", len(molecules))
+	if len(molecules) != 4 {
+		t.Errorf("expected 4 built-in molecules, got %d", len(molecules))
 	}
 
 	// Verify each molecule can be parsed and validated
@@ -141,3 +141,34 @@ func TestResearchMolecule(t *testing.T) {
 		t.Errorf("document should need investigate, got %v", steps[1].Needs)
 	}
 }
+
+func TestInstallGoBinaryMolecule(t *testing.T) {
+	mol := InstallGoBinaryMolecule()
+
+	if mol.ID != "mol-install-go-binary" {
+		t.Errorf("expected ID 'mol-install-go-binary', got %q", mol.ID)
+	}
+
+	if mol.Title != "Install Go Binary" {
+		t.Errorf("expected Title 'Install Go Binary', got %q", mol.Title)
+	}
+
+	steps, err := ParseMoleculeSteps(mol.Description)
+	if err != nil {
+		t.Fatalf("failed to parse: %v", err)
+	}
+
+	// Should have 1 step: install
+	if len(steps) != 1 {
+		t.Errorf("expected 1 step, got %d", len(steps))
+	}
+
+	if steps[0].Ref != "install" {
+		t.Errorf("expected ref 'install', got %q", steps[0].Ref)
+	}
+
+	// install has no deps
+	if len(steps[0].Needs) != 0 {
+		t.Errorf("install should have no deps, got %v", steps[0].Needs)
+	}
+}