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>

commands/gsd/discuss-phase.md

@@ -1,7 +1,7 @@
 ---
 name: gsd:discuss-phase
 description: Gather phase context through adaptive questioning before planning
-argument-hint: "<phase>"
+argument-hint: "<phase> [--auto]"
 allowed-tools:
   - Read
   - Write
@@ -9,6 +9,7 @@ allowed-tools:
   - Glob
   - Grep
   - AskUserQuestion
+  - Task
 ---
 
 <objective>

commands/gsd/plan-phase.md

@@ -1,7 +1,7 @@
 ---
 name: gsd:plan-phase
 description: Create detailed execution plan for a phase (PLAN.md) with verification loop
-argument-hint: "[phase] [--research] [--skip-research] [--gaps] [--skip-verify]"
+argument-hint: "[phase] [--auto] [--research] [--skip-research] [--gaps] [--skip-verify]"
 agent: gsd-planner
 allowed-tools:
   - Read