feat(resolvers): T2 — ResolverEntry + appliesTo gate infrastructure

Adds the conditional-resolver-injection plumbing from the v2_PLAN A.1
step. Resolvers can now be either a bare ResolverFn (always fires, current
behavior) or a ResolverEntry { resolve, appliesTo? } (gated; appliesTo
returning false skips the resolver, substitutes empty string).

Why infrastructure-only: the audit during T0a confirmed most resolvers
don't need gating. The {{NAME}} placeholder system is already conditional
at the template level — a resolver only fires for skills that reference it.
The gate is for future use when a placeholder's audience needs a structural
guardrail beyond social convention, or when a sub-resolver inside a larger
composed resolver (e.g. preamble) needs per-skill skip.

scripts/gen-skill-docs.ts:444 now uses unwrapResolver() to handle both
shapes. RESOLVERS map signature widens from Record<string, ResolverFn>
to Record<string, ResolverValue>. All existing resolvers stay bare
functions and work unchanged.

Test plan:
- bun test test/resolver-entry.test.ts: 6 pass (gate plumbing + registry)
- bun test test/gen-skill-docs.test.ts: 389 pass (no regression)
- bun run gen:skill-docs --dry-run: all SKILL.md files FRESH (no diff)

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

scripts/resolvers/index.ts

@@ -1,9 +1,20 @@
 /**
- * RESOLVERS record — maps {{PLACEHOLDER}} names to generator functions.
+ * RESOLVERS record — maps {{PLACEHOLDER}} names to generator functions
+ * or gated entries.
+ *
  * Each resolver takes a TemplateContext and returns the replacement string.
+ * Resolvers may be either a bare function (always fires) or a gated entry
+ * ({ resolve, appliesTo }) where appliesTo can return false to skip the
+ * resolver for a given skill. See ./types.ts: ResolverEntry.
+ *
+ * Most resolvers don't need a gate — the {{NAME}} placeholder system is
+ * already conditional at the template level (the resolver only fires for
+ * skills that reference it). Use a gate when you want a structural
+ * guardrail that says "this placeholder is meaningful only in skills X, Y, Z"
+ * even if someone later adds {{NAME}} to skill W.
  */
 
-import type { TemplateContext, ResolverFn } from './types';
+import type { TemplateContext, ResolverFn, ResolverValue } from './types';
 
 // Domain modules
 import { generatePreamble } from './preamble';
@@ -24,7 +35,7 @@ import { generateQuestionPreferenceCheck, generateQuestionLog, generateInlineTun
 import { generateMakePdfSetup } from './make-pdf';
 import { generateTasksSectionEmit, generateTasksSectionAggregate } from './tasks-section';
 
-export const RESOLVERS: Record<string, ResolverFn> = {
+export const RESOLVERS: Record<string, ResolverValue> = {
   SLUG_EVAL: generateSlugEval,
   SLUG_SETUP: generateSlugSetup,
   COMMAND_REFERENCE: generateCommandReference,