feat: add --discuss flag to /gsd:quick for lightweight pre-planning discussion (#861)

Surfaces gray areas and captures user decisions in CONTEXT.md before
planning, reducing hallucination risk for ambiguous quick tasks.
Composable with --full for discussion + plan-checking + verification.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

commands/gsd/quick.md

@@ -1,7 +1,7 @@
 ---
 name: gsd:quick
 description: Execute a quick task with GSD guarantees (atomic commits, state tracking) but skip optional agents
-argument-hint: "[--full]"
+argument-hint: "[--full] [--discuss]"
 allowed-tools:
   - Read
   - Write
@@ -20,9 +20,13 @@ Quick mode is the same system with a shorter path:
 - Quick tasks live in `.planning/quick/` separate from planned phases
 - Updates STATE.md "Quick Tasks Completed" table (NOT ROADMAP.md)
 
-**Default:** Skips research, plan-checker, verifier. Use when you know exactly what to do.
+**Default:** Skips research, discussion, plan-checker, verifier. Use when you know exactly what to do.
+
+**`--discuss` flag:** Lightweight discussion phase before planning. Surfaces assumptions, clarifies gray areas, captures decisions in CONTEXT.md. Use when the task has ambiguity worth resolving upfront.
 
 **`--full` flag:** Enables plan-checking (max 2 iterations) and post-execution verification. Use when you want quality guarantees without full milestone ceremony.
+
+Flags are composable: `--discuss --full` gives discussion + plan-checking + verification.
 </objective>
 
 <execution_context>