feat(settings): add /gsd:settings command for workflow toggles

Adds interactive settings command to toggle workflow agents:
- Plan Researcher (workflow.research)
- Plan Checker (workflow.plan_check)
- Execution Verifier (workflow.verifier)

Updates plan-phase.md and execute-phase.md to check config before
spawning these agents. Flags still override config per-invocation.

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

get-shit-done/templates/config.json

@@ -1,6 +1,11 @@
 {
   "mode": "interactive",
   "depth": "standard",
+  "workflow": {
+    "research": true,
+    "plan_check": true,
+    "verifier": true
+  },
   "planning": {
     "commit_docs": true,
     "search_gitignored": false