feat(admin): web-based skill management with file editor

Add full skill lifecycle to the org-admin web surface: create, read,
edit, disable. Skills are directories (SKILL.md manifest + supporting
files), content-addressed by SHA-256 in an immutable store.

Backend:
- skillStore: extract commitSkillContent (shared populate→inspect→
  dedup→atomic rename); add importSkillFromFiles (in-memory file list
  ingestion) and readSkillFiles (read stored version back as UTF-8)
- configuration: add installSkillFromFiles, readSkillFiles, disableSkill
  (soft-delete + archive bound role sessions), updateSkillDescription
  (label-only, no archival); refactor installSkill to share
  commitInstalledSkill
- agentConfigRoutes: wire skillStoreRoot; add GET
  /agent-skills/:name/files, PUT /agent-skills/:name (create/replace),
  PATCH /agent-skills/:name (description/disable)
- orgRoutes: pass readSkillStoreRoot() to agent config routes

Frontend:
- api.ts: agentSkillFiles, installAgentSkill, patchAgentSkill methods
- SkillEditor.svelte: file tree + text editor + version/description form
- skills/+page.svelte: skill list, create form (generates SKILL.md
  template), per-skill editor
- layout: add 技能 nav item

ADR-0018: update Decision to reflect web surface joining host-console
CLI in the shared content-addressed ingestion pipeline.

Spec (AgentRole.lean): unchanged — storage mechanism is OPEN, web
installation is one implementation of it.

hub/admin-web/src/lib/api.ts

@@ -253,6 +253,17 @@ export interface AgentSkillRow {
 	boundRoleIds: readonly string[];
 }
 
+export interface SkillFileEntry {
+	path: string;
+	content: string;
+}
+
+export interface InstalledSkillResult {
+	id: string;
+	name: string;
+	contentDigest: string;
+}
+
 export interface AgentModelRow {
 	id: string;
 	label: string;
@@ -392,5 +403,11 @@ export const api = {
 			skillNames: string[];
 		}>,
 	agentSkills: (slug: string) => get(`${orgBase(slug)}/agent-skills`) as Promise<{ skills: AgentSkillRow[] }>,
+	agentSkillFiles: (slug: string, name: string) =>
+		get(`${orgBase(slug)}/agent-skills/${encodeURIComponent(name)}/files`) as Promise<{ files: SkillFileEntry[] }>,
+	installAgentSkill: (slug: string, name: string, body: { version: string; files: readonly SkillFileEntry[] }) =>
+		put(`${orgBase(slug)}/agent-skills/${encodeURIComponent(name)}`, body) as Promise<InstalledSkillResult>,
+	patchAgentSkill: (slug: string, name: string, body: { description?: string; disabled?: boolean }) =>
+		patch(`${orgBase(slug)}/agent-skills/${encodeURIComponent(name)}`, body) as Promise<{ disabled?: boolean; updated?: boolean }>,
 	agentModels: (slug: string) => get(`${orgBase(slug)}/agent-models`) as Promise<{ models: AgentModelRow[] }>,
 };