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/core-api/src/system_bus.rs

@@ -105,6 +105,21 @@ pub enum SystemEvent {
         catalog_name: String,
     },
 
+    // ── Skills (blueprint skill-project §8) ───────────────────────────────────
+    /// A skills tree changed on disk **in a way the index feels** — a skill was
+    /// added, removed or re-described by someone editing files by hand on the
+    /// box. Emitted by the freshness watcher after its digest gate: a change
+    /// that leaves the index byte-identical (a script, a reference document)
+    /// announces nothing, because the frozen system prefix citing that skill
+    /// has not aged. The in-process writers (`skill_register`/`skill_delete`)
+    /// never emit this — they invalidate directly.
+    ///
+    /// Pure reconciliation, the contract this bus already promises: a lost
+    /// event costs a stale skill index for the prefix TTL, never a wrong one.
+    SkillsChanged {
+        scope: SkillScope,
+    },
+
     // ── Reports (blueprint §13) ───────────────────────────────────────────────
     /// A background agent filed a report. Announced by whoever wrote the row,
     /// never delivered by it: *who* should hear about a report — the people
@@ -125,6 +140,19 @@ pub enum SystemEvent {
 
 // ── Bus ───────────────────────────────────────────────────────────────────────
 
+/// Which skills tree a [`SystemEvent::SkillsChanged`] is about.
+///
+/// Distinct from the `"mine" | "global"` vocabulary of the skill tools: this
+/// names a *place on disk*, and a change to the group's tree concerns every
+/// member's prompt while a change to one member's tree concerns only theirs.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SkillScope {
+    /// `{WD}/skills` — the group's tree, in every member's index.
+    Global,
+    /// `{WD}/skills-users/{userid}` — one member's own tree.
+    User(String),
+}
+
 pub struct SystemEventBus {
     tx: broadcast::Sender<SystemEvent>,
 }