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.