feat(new-project): add --auto flag for unattended initialization

Runs research → requirements → roadmap automatically after config
questions. Requires idea document via @ reference. Auto-includes all
table stakes features plus features mentioned in provided document.

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

commands/gsd/new-project.md

@@ -1,6 +1,7 @@
 ---
 name: gsd:new-project
 description: Initialize a new project with deep context gathering and PROJECT.md
+argument-hint: "[--auto]"
 allowed-tools:
   - Read
   - Bash
@@ -8,6 +9,11 @@ allowed-tools:
   - Task
   - AskUserQuestion
 ---
+<context>
+**Flags:**
+- `--auto` — Automatic mode. After config questions, runs research → requirements → roadmap without further interaction. Expects idea document via @ reference.
+</context>
+
 <objective>
 Initialize a new project through unified flow: questioning → research (optional) → requirements → roadmap.