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/routes/+layout.svelte

@@ -25,6 +25,7 @@
 		{ key: 'projects', label: '项目', icon: 'projects' as const },
 		{ key: 'capacity', label: '容量', icon: 'overview' as const },
 		{ key: 'provider', label: '供应方', icon: 'provider' as const },
+		{ key: 'skills', label: '技能', icon: 'roles' as const },
 		{ key: 'roles', label: '角色', icon: 'roles' as const },
 		{ key: 'feishu', label: '飞书', icon: 'feishu' as const },
 	];