feat(10): add parallelization question to /gsd:new-project

- New step after depth selection
- Options: Enabled (recommended) / Disabled
- Config.json now populated with mode, depth, and parallelization

commands/gsd/new-project.md

@@ -248,7 +248,23 @@ Use AskUserQuestion:
 
 **Depth controls compression tolerance, not artificial inflation.** All depths use 2-3 tasks per plan. Comprehensive means "don't compress complex work"—it doesn't mean "pad simple work to hit a number."
 
-Create `.planning/config.json` with chosen mode and depth using `templates/config.json` structure.
+</step>
+
+<step name="parallelization">
+
+Ask parallel execution preference:
+
+Use AskUserQuestion:
+
+- header: "Parallelization"
+- question: "Enable parallel phase execution?"
+- options:
+  - "Enabled" — Run independent plans in parallel (Recommended)
+  - "Disabled" — Execute plans sequentially
+
+**Parallelization allows `/gsd:execute-phase` to spawn multiple agents for independent plans.** Useful for "walk away, come back to completed work" execution. Can be changed later in config.json.
+
+Create `.planning/config.json` with chosen mode, depth, and parallelization using `templates/config.json` structure.
 
 </step>
 
@@ -309,7 +325,7 @@ Project initialized:
 - [ ] PROJECT.md captures full context with evolutionary structure
 - [ ] Requirements initialized as hypotheses (greenfield) or with inferred Validated (brownfield)
 - [ ] Key Decisions table initialized
-- [ ] config.json has workflow mode
+- [ ] config.json has workflow mode, depth, and parallelization
 - [ ] All committed to git
 
 </success_criteria>