feat: add INVOKE_SKILL resolver for composable skill loading

New composition.ts resolver module that emits prose instructing Claude
to read another skill's SKILL.md and follow it, skipping preamble
sections. Supports optional skip= parameter for additional sections.

Usage: {{INVOKE_SKILL:plan-ceo-review}} or
       {{INVOKE_SKILL:plan-ceo-review:skip=Outside Voice}}

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

scripts/resolvers/composition.ts

@@ -0,0 +1,48 @@
+import type { TemplateContext } from './types';
+
+/**
+ * {{INVOKE_SKILL:skill-name}} — emits prose instructing Claude to read
+ * another skill's SKILL.md and follow it, skipping preamble sections.
+ *
+ * Supports optional skip= parameter for additional sections to skip:
+ *   {{INVOKE_SKILL:plan-ceo-review:skip=Outside Voice,Design Outside Voices}}
+ */
+export function generateInvokeSkill(ctx: TemplateContext, args?: string[]): string {
+  const skillName = args?.[0];
+  if (!skillName || skillName === '') {
+    throw new Error('{{INVOKE_SKILL}} requires a skill name, e.g. {{INVOKE_SKILL:plan-ceo-review}}');
+  }
+
+  // Parse optional skip= parameter from args[1+]
+  const extraSkips = (args?.slice(1) || [])
+    .filter(a => a.startsWith('skip='))
+    .flatMap(a => a.slice(5).split(','))
+    .map(s => s.trim())
+    .filter(Boolean);
+
+  const DEFAULT_SKIPS = [
+    'Preamble (run first)',
+    'AskUserQuestion Format',
+    'Completeness Principle — Boil the Lake',
+    'Search Before Building',
+    'Contributor Mode',
+    'Completion Status Protocol',
+    'Telemetry (run last)',
+    'Step 0: Detect platform and base branch',
+    'Review Readiness Dashboard',
+    'Plan File Review Report',
+    'Prerequisite Skill Offer',
+    'Plan Status Footer',
+  ];
+
+  const allSkips = [...DEFAULT_SKIPS, ...extraSkips];
+
+  return `Read the \`/${skillName}\` skill file at \`${ctx.paths.skillRoot}/${skillName}/SKILL.md\` using the Read tool.
+
+**If unreadable:** Skip with "Could not load /${skillName} — skipping." and continue.
+
+Follow its instructions from top to bottom, **skipping these sections** (already handled by the parent skill):
+${allSkips.map(s => `- ${s}`).join('\n')}
+
+Execute every other section at full depth. When the loaded skill's instructions are complete, continue with the next step below.`;
+}