Add gate intensity modes: full, lean, and solo review control

Adds a --review flag to all 13 gate-using skills and a global
production/review-mode.txt config (set during /start). Full runs all
director gates, lean skips per-skill gates but keeps phase gates, solo
skips all directors. Documents modes in director-gates.md, WORKFLOW-GUIDE.md,
and README.md.

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

.claude/skills/brainstorm/SKILL.md

@@ -1,7 +1,7 @@
 ---
 name: brainstorm
 description: "Guided game concept ideation — from zero idea to a structured game concept document. Uses professional studio ideation techniques, player psychology frameworks, and structured creative exploration."
-argument-hint: "[genre or theme hint, or 'open' for fully open brainstorm]"
+argument-hint: "[genre or theme hint, or 'open'] [--review full|lean|solo]"
 user-invocable: true
 allowed-tools: Read, Glob, Grep, Write, WebSearch, AskUserQuestion
 ---
@@ -10,7 +10,8 @@ When this skill is invoked:
 
 1. **Parse the argument** for an optional genre/theme hint (e.g., `roguelike`,
    `space survival`, `cozy farming`). If `open` or no argument, start from
-   scratch.
+   scratch. Also extract `--review [full|lean|solo]` if present and store as
+   the review mode override for this run (see `.claude/docs/director-gates.md`).
 
 2. **Check for existing concept work**:
    - Read `design/gdd/game-concept.md` if it exists (resume, don't restart)