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>

commands/gsd/settings.md

@@ -0,0 +1,122 @@
+---
+name: gsd:settings
+description: Configure GSD workflow toggles (research, plan checker, verifier)
+allowed-tools:
+  - Read
+  - Write
+  - AskUserQuestion
+---
+
+<objective>
+Allow users to toggle workflow agents on/off via interactive settings.
+
+Updates `.planning/config.json` with workflow preferences that control whether research, plan checking, and verification agents are spawned.
+</objective>
+
+<process>
+
+## 1. Validate Environment
+
+```bash
+ls .planning/config.json 2>/dev/null
+```
+
+**If not found:** Error - run `/gsd:new-project` first.
+
+## 2. Read Current Config
+
+```bash
+cat .planning/config.json
+```
+
+Parse current values (default to `true` if not present):
+- `workflow.research` — spawn researcher during plan-phase
+- `workflow.plan_check` — spawn plan checker during plan-phase
+- `workflow.verifier` — spawn verifier during execute-phase
+
+## 3. Present Settings
+
+Use AskUserQuestion with current values shown:
+
+```
+AskUserQuestion([
+  {
+    question: "Spawn Plan Researcher? (researches domain before planning)",
+    header: "Research",
+    multiSelect: false,
+    options: [
+      { label: "Yes", description: "Research phase goals before planning" },
+      { label: "No", description: "Skip research, plan directly" }
+    ]
+  },
+  {
+    question: "Spawn Plan Checker? (verifies plans before execution)",
+    header: "Plan Check",
+    multiSelect: false,
+    options: [
+      { label: "Yes", description: "Verify plans meet phase goals" },
+      { label: "No", description: "Skip plan verification" }
+    ]
+  },
+  {
+    question: "Spawn Execution Verifier? (verifies phase completion)",
+    header: "Verifier",
+    multiSelect: false,
+    options: [
+      { label: "Yes", description: "Verify must-haves after execution" },
+      { label: "No", description: "Skip post-execution verification" }
+    ]
+  }
+])
+```
+
+**Pre-select based on current config values.**
+
+## 4. Update Config
+
+Merge new workflow settings into existing config.json:
+
+```json
+{
+  ...existing_config,
+  "workflow": {
+    "research": true/false,
+    "plan_check": true/false,
+    "verifier": true/false
+  }
+}
+```
+
+Write updated config to `.planning/config.json`.
+
+## 5. Confirm Changes
+
+Display:
+
+```
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+ GSD ► SETTINGS UPDATED
+━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
+
+| Setting              | Value |
+|----------------------|-------|
+| Plan Researcher      | {On/Off} |
+| Plan Checker         | {On/Off} |
+| Execution Verifier   | {On/Off} |
+
+These settings apply to future /gsd:plan-phase and /gsd:execute-phase runs.
+
+Override per-invocation with flags:
+- /gsd:plan-phase --research (force research)
+- /gsd:plan-phase --skip-research (skip research)
+- /gsd:plan-phase --skip-verify (skip plan check)
+```
+
+</process>
+
+<success_criteria>
+- [ ] Current config read
+- [ ] User presented with 3 toggle questions
+- [ ] Config updated with workflow section
+- [ ] Changes confirmed to user
+</success_criteria>