feat: DESIGN_SETUP + DESIGN_MOCKUP template resolvers

Add generateDesignSetup() and generateDesignMockup() to the existing
design.ts resolver file. Add designDir to HostPaths (claude + codex).
Register DESIGN_SETUP and DESIGN_MOCKUP in the resolver index.

DESIGN_SETUP: $D binary discovery (mirrors $B browse setup pattern).
Falls back to DESIGN_SKETCH if binary not available.

DESIGN_MOCKUP: full visual exploration workflow template — construct
brief from DESIGN.md context, generate 3 variants, open comparison
board in Chrome, poll for user feedback, save approved mockup to
docs/designs/, generate HTML wireframe for implementation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/resolvers/types.ts

@@ -5,6 +5,7 @@ export interface HostPaths {
   localSkillRoot: string;
   binDir: string;
   browseDir: string;
+  designDir: string;
 }
 
 export const HOST_PATHS: Record<Host, HostPaths> = {
@@ -13,12 +14,14 @@ export const HOST_PATHS: Record<Host, HostPaths> = {
     localSkillRoot: '.claude/skills/gstack',
     binDir: '~/.claude/skills/gstack/bin',
     browseDir: '~/.claude/skills/gstack/browse/dist',
+    designDir: '~/.claude/skills/gstack/design/dist',
   },
   codex: {
     skillRoot: '$GSTACK_ROOT',
     localSkillRoot: '.agents/skills/gstack',
     binDir: '$GSTACK_BIN',
     browseDir: '$GSTACK_BROWSE',
+    designDir: '$GSTACK_DESIGN',
   },
 };