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.

.gitignore

@@ -53,8 +53,16 @@ node_modules/
 # ── macOS ─────────────────────────────────────────────────────────────────────
 .DS_Store
 
-# ── Private skills ────────────────────────────────────────────────────────────
-skills/.gitignore
+# ── Skills (blueprint: skill system) ──────────────────────────────────────────
+# The build ships no skills: every one of these directories is instance data,
+# filled only by what a member registers. `skills/` is the group-wide tree,
+# `skills-users/{userid}/` a member's own, and `.skills-root/{userid}/` the
+# read-only mount that carries the signpost plus the two scope mountpoints
+# (regenerated at every container `ensure` from the consts in
+# crates/skald-core/src/container/mod.rs).
+/skills/
+/skills-users/
+/.skills-root/
 
 # ── Editors & IDEs ────────────────────────────────────────────────────────────
 .claude/