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.

docs/adr/0018-agent-execution-surface-bounded-by-workspace.md

@@ -123,15 +123,23 @@ The boundary is enforced by the Claude Code SDK's built-in sandbox
   workspace or service-user config from widening tools, hooks, MCP servers, or
   sandbox paths.
 - Agent skills are Organization-scoped runtime configuration, not Hub release
-  assets. A controlled host-console installer imports each version into a
-  content-addressed persistent store and records its digest in PostgreSQL. A
-  role selects enabled Organization skills alongside its model, system prompt
-  and tool allowlist. Each run copies only those selected immutable versions
-  into a run-scoped plugin outside the project workspace; the sandbox exposes
-  that snapshot read-only and deletes it after the run. SDK-bundled skills and
-  filesystem setting sources remain disabled, so project `.claude` content
-  cannot register skills or widen tools. Requested skill versions are recorded
-  on `run.created`; SDK initialization remains authoritative loading evidence.
+  assets. Skill content is imported into a content-addressed persistent store
+  through a shared ingestion pipeline (`importSkillDirectory` for host-console
+  CLI, `importSkillFromFiles` for org-admin web surface) that enforces the same
+  safety checks: `SKILL.md` manifest required, 512-file / 16-byte limits,
+  symlink rejection, SHA-256 content addressing. The web surface
+  (`OrganizationAgentConfiguration.installSkillFromFiles`) writes to the same
+  store as the host-console CLI (`installSkill`); both flow through
+  `commitSkillContent` for deduplication and atomic rename into
+  `versions/<digest>/`. Org-admin authentication gates the web surface; the
+  content-addressed store remains platform-controlled. A role selects enabled
+  Organization skills alongside its model, system prompt and tool allowlist.
+  Each run copies only those selected immutable versions into a run-scoped
+  plugin outside the project workspace; the sandbox exposes that snapshot
+  read-only and deletes it after the run. SDK-bundled skills and filesystem
+  setting sources remain disabled, so project `.claude` content cannot register
+  skills or widen tools. Requested skill versions are recorded on
+  `run.created`; SDK initialization remains authoritative loading evidence.
 - Network: open (see Open Questions).
 
 `bypassPermissions` is kept (headless server — no interactive prompts); the