feat(skills): rebuild the skill system for the multi-user model

Per blueprint/skill-project.md: the old single-namespace, hand-maintained
index is gone, replaced by a read-only, two-scope tree whose index is a
runtime function of its content.

- skills/ index generated at runtime (crates/skald-core/src/skills/:
  inventory, install, validate, watch), injected through the new
  <!-- SKILLS_LIST --> placeholder in AGENT.md (agents/common/skills.md);
  meta.json inject_skills flag removed. 11 chat/task agents carry the
  include, the 4 system agents do not.
- Two trees, both read-only in both directions: skills/shared/{id} (the
  group's) and skills/{username}/{id} (one member's own, on the stable
  userid). The root is closed too: UserFs::SkillMounts + RouteError (alias
  probe, plain-denied paths, no home fallback) and a per-user
  .skills-root/{userid} container mount with the two scope mounts nested
  inside, plus the fifth self-heal axis (skills_mounted).
- Agent verbs: skill_register/skill_delete (Config group, global scope
  behind the new skill.manage capability), fetch_repo for public repos,
  list_items(type="skills"); reads are plain read_file on the printed
  path. Seeded @fs_read skills/* allow.
- Freshness: a digest-gated watcher on the two trees emits
  SystemEvent::SkillsChanged, whose subscriber rebuilds the frozen prompt
  prefix via Skald::invalidate_prompt_prefix; in-process writers invalidate
  directly.
- The build ships no skills: the three bundled skills (ics2json,
  mcp-builder, skill-creator) and skills/index.md are removed, skills/ is
  instance data (gitignored, not packaged, no longer pruned by update.sh).
- Docs: skills.md, agents.md, shared-folders.md added; docs/index.md and
  agents/README.md updated.

crates/skald-core/src/db/role_capabilities.rs

@@ -36,6 +36,19 @@ pub const MANAGE_SHARED_FOLDERS: &str = "folders.manage";
 /// pattern as [`MANAGE_SHARED_FOLDERS`].
 pub const MANAGE_PLUGINS: &str = "plugin.manage";
 
+/// Install or delete a skill in the **group's** tree — `skill_register`/
+/// `skill_delete` with `scope: "global"` (blueprint §7.3/§9). One's own scope
+/// needs no capability: it is the caller's, always.
+///
+/// Deliberately **not** in [`DEFAULT_USER_CAPABILITIES`], unlike the two
+/// self-service MCP ones, and the asymmetry is the point: a global skill is text
+/// that enters every member's prompt and is read there as an instruction, so it
+/// is closer to curating the catalog than to activating a connector for oneself.
+/// `admin` therefore holds it implicitly (via [`has`]) and opening it to another
+/// role later is a single [`grant`], no code change — the same shape as
+/// [`MANAGE_SHARED_FOLDERS`] and [`MANAGE_PLUGINS`].
+pub const MANAGE_SKILLS: &str = "skill.manage";
+
 /// The default capabilities of an ordinary (non-admin) user role.
 pub const DEFAULT_USER_CAPABILITIES: &[&str] = &[REGISTER_REMOTE, REGISTER_LOCAL_FROM_CATALOG];