feat(workflow): add auto-advance pipeline (--auto flag + workflow.auto_advance config)

Add opt-in mechanism to chain discuss → plan → execute automatically
via Task() subagents, eliminating manual /clear + paste overhead.

- Add `--auto` flag to discuss-phase and plan-phase commands
- Add `workflow.auto_advance` config toggle (default: false)
- Add auto_advance step to discuss-phase workflow (spawns plan-phase)
- Add step 14 to plan-phase workflow (spawns execute-phase)
- Add auto_advance toggle to /gsd:settings

Chain stops gracefully on INCONCLUSIVE, CHECKPOINT, or verification
failures. No work lost — artifacts persist at each step.

Closes #541

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

get-shit-done/workflows/plan-phase.md

@@ -319,7 +319,59 @@ Offer: 1) Force proceed, 2) Provide guidance and retry, 3) Abandon
 
 ## 13. Present Final Status
 
-Route to `<offer_next>`.
+Route to `<offer_next>` OR `auto_advance` depending on flags/config.
+
+## 14. Auto-Advance Check
+
+Check for auto-advance trigger:
+
+1. Parse `--auto` flag from $ARGUMENTS
+2. Read `workflow.auto_advance` from config:
+   ```bash
+   AUTO_CFG=$(node ~/.claude/get-shit-done/bin/gsd-tools.js config get workflow.auto_advance 2>/dev/null || echo "false")
+   ```
+
+**If `--auto` flag present OR `AUTO_CFG` is true:**
+
+Display banner:
+```
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+ GSD ► AUTO-ADVANCING TO EXECUTE
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+Plans ready. Spawning execute-phase...
+```
+
+Spawn execute-phase as Task:
+```
+Task(
+  prompt="Run /gsd:execute-phase ${PHASE}",
+  subagent_type="general-purpose",
+  description="Execute Phase ${PHASE}"
+)
+```
+
+**Handle execute-phase return:**
+- **PHASE COMPLETE** → Display final summary:
+  ```
+  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+   GSD ► PHASE ${PHASE} COMPLETE ✓
+  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+  Auto-advance pipeline finished.
+
+  Next: /gsd:discuss-phase ${NEXT_PHASE} --auto
+  ```
+- **GAPS FOUND / VERIFICATION FAILED** → Display result, stop chain:
+  ```
+  Auto-advance stopped: Execution needs review.
+
+  Review the output above and continue manually:
+  /gsd:execute-phase ${PHASE}
+  ```
+
+**If neither `--auto` nor config enabled:**
+Route to `<offer_next>` (existing behavior).
 
 </process>