feat(skills): single-source skill system with markdown SKILL.md + include directive

Skills move out of packages/core/src/application/assistant/skills/*/skill.ts
(TS string constants) into apps/skills/<id>/SKILL.md (Agent Skills spec format
— YAML frontmatter + markdown body). One directory, one loader, one place to
look at every skill the agent can load.

Key change vs the old dev system: a `{{include:<skill-id>}}` directive lets one
skill transclude another. This removes the parallel TS constant for the
knowledge-note style guide — it now lives at apps/skills/knowledge-note-style/
(hidden from catalog) and is pulled into doc-collab + the live-note and
background-task agents via the resolver instead of via a TS import.

Infrastructure:
- packages/core/src/skills/ — types, skill-md-parser, FS-backed official repo,
  SkillResolver with recursive {{include:<id>}} expansion + cycle detection
- packages/shared/src/skill.ts — SkillFrontmatter, SkillCatalogEntry,
  ResolvedSkill schemas
- DI: officialSkillsRepo + skillResolver registered; registerSkillsDir helper
  wires the path before any consumer resolves
- IPC: skills:list / skills:get (read-only) for the Settings UI
- Main: resolveSkillsDir picks Resources/skills (packaged) or repo apps/skills
  (dev). forge.config.cjs ships apps/skills/ as extraResource.

Consumer refactor:
- buildCopilotInstructions: catalog markdown built from resolver.getCatalog()
- builtin-tools: loadSkill uses resolver, new listSkills tool
- background-tasks/agent + live-note/agent: now async builders that load
  the knowledge-note-style skill content via resolver
- runtime.loadAgent: awaits the now-async builders
- Deleted: assistant/skills/ directory, knowledge-note-style.ts

UI:
- New SkillsSettings component (read-only list + detail view) wired into
  Settings dialog as the "Skills" tab.

apps/x/packages/shared/src/index.ts

@@ -16,4 +16,5 @@ export * as promptBlock from './prompt-block.js';
 export * as frontmatter from './frontmatter.js';
 export * as bases from './bases.js';
 export * as browserControl from './browser-control.js';
+export * as skill from './skill.js';
 export { PrefixLogger };

apps/x/packages/shared/src/ipc.ts

@@ -17,6 +17,7 @@ import { UserMessageContent } from './message.js';
 import { RowboatApiConfig } from './rowboat-account.js';
 import { ZListToolkitsResponse } from './composio.js';
 import { BrowserStateSchema } from './browser-control.js';
+import { ResolvedSkill, SkillCatalogEntry } from './skill.js';
 
 // ============================================================================
 // Runtime Validation Schemas (Single Source of Truth)
@@ -871,6 +872,17 @@ const ipcSchemas = {
     req: BrowserStateSchema,
     res: z.null(),
   },
+  // Skills channels (read-only)
+  'skills:list': {
+    req: z.null(),
+    res: z.object({
+      skills: z.array(SkillCatalogEntry),
+    }),
+  },
+  'skills:get': {
+    req: z.object({ id: z.string() }),
+    res: ResolvedSkill.nullable(),
+  },
   // Billing channels
   'billing:getInfo': {
     req: z.null(),

apps/x/packages/shared/src/skill.ts

@@ -0,0 +1,36 @@
+import { z } from 'zod';
+
+// SKILL.md frontmatter schema. `name` is the skill id (folder name) and
+// `description` is the one-line catalog summary. `hidden: true` keeps a
+// skill out of the public catalog while still allowing other skills to
+// `{{include:<id>}}` it as content (e.g. shared style guides).
+export const SkillFrontmatter = z.object({
+    name: z.string().max(64),
+    description: z.string().max(1024),
+    hidden: z.boolean().optional(),
+    license: z.string().optional(),
+    metadata: z.object({
+        title: z.string().optional(),
+    }).passthrough().optional(),
+});
+
+export type SkillFrontmatter = z.infer<typeof SkillFrontmatter>;
+
+// Skill catalog entry seen by the agent and the renderer (no content body).
+export const SkillCatalogEntry = z.object({
+    id: z.string(),
+    title: z.string(),
+    summary: z.string(),
+});
+export type SkillCatalogEntry = z.infer<typeof SkillCatalogEntry>;
+
+// Fully-resolved skill: catalog metadata + body with all {{include:<id>}}
+// directives expanded.
+export const ResolvedSkill = z.object({
+    id: z.string(),
+    title: z.string(),
+    summary: z.string(),
+    content: z.string(),
+});
+
+export type ResolvedSkill = z.infer<typeof ResolvedSkill>;