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.

ci/package.sh

@@ -16,7 +16,7 @@
 #   --output     Directory where the .tar.gz will be written
 #
 # The tarball contains everything needed to run (or uninstall) Skald Circle:
-#   bin/skald, bin/skald-setup, web/, agents/, commands/, skills/, docs/,
+#   bin/skald, bin/skald-setup, web/, agents/, commands/, docs/,
 #   default.config.yaml, providers.yaml, requirements.txt,
 #   requirements-optional.txt, run.sh, update.sh, uninstall.sh
 
@@ -93,7 +93,8 @@ chmod 755 "$STAGING/bin/skald" "$STAGING/bin/skald-setup"
 cp -r web              "$STAGING/web"
 cp -r agents           "$STAGING/agents"
 cp -r commands         "$STAGING/commands"
-cp -r skills           "$STAGING/skills"
+# No `skills/`: the build ships no skills (they are instance data, registered by
+# members), so the directory is created by the app, never by the tarball.
 cp -r docs             "$STAGING/docs"
 cp default.config.yaml "$STAGING/default.config.yaml"
 cp providers.yaml      "$STAGING/providers.yaml"