feat: Add gt mol step done for auto-continuing molecule steps (gt-5gq8r)

Implements the canonical way for polecats to complete molecule steps:
1. Closes the completed step (bd close)
2. Extracts molecule ID from step ID (gt-xxx.1 -> gt-xxx)
3. Finds next ready step (dependency-aware)
4. If next step: updates hook and respawns pane
5. If complete: burns hook and signals witness

This enables instant step transitions (~5-10s) vs waiting for witness
patrol cycles (minutes), and ensures the activity feed stays accurate.

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

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

internal/cmd/molecule.go

@@ -305,6 +305,23 @@ a permanent (but compact) record.`,
 	RunE: runMoleculeSquash,
 }
 
+var moleculeStepCmd = &cobra.Command{
+	Use:   "step",
+	Short: "Molecule step operations",
+	Long: `Commands for working with molecule steps.
+
+A molecule is a DAG of steps. Each step is a beads issue with the molecule root
+as its parent. Steps can have dependencies on other steps.
+
+When a polecat is working on a molecule, it processes one step at a time:
+1. Work on the current step
+2. When done: gt mol step done <step-id>
+3. System auto-continues to next ready step
+
+IMPORTANT: Always use 'gt mol step done' to complete steps. Do not manually
+close steps with 'bd close' - that skips the auto-continuation logic.`,
+}
+
 var moleculeBondCmd = &cobra.Command{
 	Use:   "bond <proto-id>",
 	Short: "Dynamically bond a child molecule to a running parent",
@@ -381,6 +398,10 @@ func init() {
 	moleculeBondCmd.Flags().BoolVar(&moleculeJSON, "json", false, "Output as JSON")
 	moleculeBondCmd.MarkFlagRequired("parent")
 
+	// Add step subcommand with its children
+	moleculeStepCmd.AddCommand(moleculeStepDoneCmd)
+	moleculeCmd.AddCommand(moleculeStepCmd)
+
 	// Add subcommands
 	moleculeCmd.AddCommand(moleculeStatusCmd)
 	moleculeCmd.AddCommand(moleculeCurrentCmd)