feat: model overlays with explicit --model flag (no auto-detect)

Adds a per-model behavioral patch layer orthogonal to the host axis.
Different LLMs have different tendencies (GPT won't stop, Gemini
over-explains, o-series wants structured output). Overlays nudge each
model toward better defaults for gstack workflows.

Codex review caught three landmines the prior reviews missed:
1. Host != model — Claude Code can run any Claude model, Codex runs
   GPT/o-series, Cursor fronts multiple providers. Auto-detecting from
   host would lie. Dropped auto-detect. --model is explicit (default
   claude). Missing overlay file → empty string (graceful).
2. Import cycle — putting Model in resolvers/types.ts would cycle
   through hosts/index. Created neutral scripts/models.ts instead.
3. "Final say" is dangerous — overlay at the end of preamble could
   override STOP points, AskUserQuestion gates, /ship review gates.
   Placed overlay after spawned-session-check but before voice + tier
   sections. Wrapper heading adds explicit subordination language on
   every overlay: "subordinate to skill workflow, STOP points,
   AskUserQuestion gates, plan-mode safety, and /ship review gates."

Changes:
- scripts/models.ts: new neutral module. ALL_MODEL_NAMES, Model type,
  resolveModel() for family heuristics (gpt-5.4-mini → gpt-5.4, o3 →
  o-series, claude-opus-4-7 → claude), validateModel() helper.
- scripts/resolvers/types.ts: import Model, add ctx.model field.
- scripts/resolvers/model-overlay.ts: new resolver. Reads
  model-overlays/{model}.md. Supports {{INHERIT:base}} directive at
  top of file for concat (gpt-5.4 inherits gpt). Cycle guard.
- scripts/resolvers/index.ts: register MODEL_OVERLAY resolver.
- scripts/resolvers/preamble.ts: wire generateModelOverlay into
  composition before voice. Print MODEL_OVERLAY: {model} in preamble
  bash so users can see which overlay is active. Filter empty sections.
- scripts/gen-skill-docs.ts: parse --model CLI flag. Default claude.
  Unknown model → throw with list of valid options.
- model-overlays/{claude,gpt,gpt-5.4,gemini,o-series}.md: behavioral
  patches per model family. gpt-5.4.md uses {{INHERIT:gpt}} to extend
  gpt.md without duplication.
- test/gen-skill-docs.test.ts: fix qa-only guardrail regex scope.
  Was matching Edit/Glob/Grep anywhere after `allowed-tools:` in the
  whole file. Now scoped to frontmatter only. Body prose (Claude
  overlay references Edit as a tool) correctly no longer breaks it.

Verification:
- bun run gen:skill-docs --host all --dry-run → all fresh
- bun run gen:skill-docs --model gpt-5.4 → concat works, gpt.md +
  gpt-5.4.md content appears in order
- bun run gen:skill-docs --model unknown → errors with valid list
- All generated skills contain MODEL_OVERLAY: claude in preamble
- Golden ship fixtures regenerated

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/gen-skill-docs.ts

@@ -43,6 +43,21 @@ const HOST_ARG_VAL: HostArg = (() => {
 // For single-host mode, HOST is the host. For --host all, it's set per iteration below.
 let HOST: Host = HOST_ARG_VAL === 'all' ? 'claude' : HOST_ARG_VAL;
 
+// ─── Model Overlay Selection ────────────────────────────────
+// --model is explicit. We do NOT auto-detect from host (host ≠ model).
+// Default is 'claude'. Missing overlay file → empty string (graceful).
+import { ALL_MODEL_NAMES, resolveModel, type Model } from './models';
+const MODEL_ARG = process.argv.find(a => a.startsWith('--model'));
+const MODEL_ARG_VAL: Model = (() => {
+  if (!MODEL_ARG) return 'claude';
+  const val = MODEL_ARG.includes('=') ? MODEL_ARG.split('=')[1] : process.argv[process.argv.indexOf(MODEL_ARG) + 1];
+  const resolved = resolveModel(val);
+  if (!resolved) {
+    throw new Error(`Unknown model: ${val}. Use ${ALL_MODEL_NAMES.join(', ')}, or a family variant (e.g., claude-opus-4-7, gpt-5.4-mini, o3).`);
+  }
+  return resolved;
+})();
+
 // HostPaths, HOST_PATHS, and TemplateContext imported from ./resolvers/types (line 7-8)
 // Design constants (AI_SLOP_BLACKLIST, OPENAI_HARD_REJECTIONS, OPENAI_LITMUS_CHECKS)
 // live in ./resolvers/constants and are consumed by resolvers directly.
@@ -398,7 +413,7 @@ function processTemplate(tmplPath: string, host: Host = 'claude'): { outputPath:
   const tierMatch = tmplContent.match(/^preamble-tier:\s*(\d+)$/m);
   const preambleTier = tierMatch ? parseInt(tierMatch[1], 10) : undefined;
 
-  const ctx: TemplateContext = { skillName, tmplPath, benefitsFrom, host, paths: HOST_PATHS[host], preambleTier };
+  const ctx: TemplateContext = { skillName, tmplPath, benefitsFrom, host, paths: HOST_PATHS[host], preambleTier, model: MODEL_ARG_VAL };
 
   // Replace placeholders (supports parameterized: {{NAME:arg1:arg2}})
   // Config-driven: suppressedResolvers return empty string for this host