feat(quick): make --full include all phases, add --validate flag

--full now enables discussion + research + plan-checking + verification.
New --validate flag covers what --full previously did (plan-checking +
verification only). All downstream workflow logic uses $VALIDATE_MODE.

Closes #1498

Co-Authored-By: Claude Opus 4.6 (1M context) <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] [--discuss] [--research]"
+argument-hint: "[--full] [--validate] [--discuss] [--research]"
 allowed-tools:
   - Read
   - Write
@@ -24,11 +24,13 @@ Quick mode is the same system with a shorter path:
 
 **`--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.
+**`--full` flag:** Enables the complete quality pipeline — discussion + research + plan-checking + verification. One flag for everything.
+
+**`--validate` flag:** Enables plan-checking (max 2 iterations) and post-execution verification only. Use when you want quality guarantees without discussion or research.
 
 **`--research` flag:** Spawns a focused research agent before planning. Investigates implementation approaches, library options, and pitfalls for the task. Use when you're unsure of the best approach.
 
-Flags are composable: `--discuss --research --full` gives discussion + research + plan-checking + verification.
+Granular flags are composable: `--discuss --research --validate` gives the same result as `--full`.
 </objective>
 
 <execution_context>