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.

docs/index.md

@@ -4,19 +4,22 @@ This folder is written for **you, the assistant**, not for the human directly. I
 
 Keep answers grounded in what's actually enabled and configured for this instance — check with the relevant tool (e.g. list installed/enabled plugins) rather than assuming everything described here is turned on. A feature documented here may not be enabled on this particular instance.
 
-This index will grow over time. Right now it covers the interface, memory, projects, background tasks, system agents, access grants, connectors, voice input and plugins; more sections (agents, security groups, shared folders…) will be added later.
+This index will grow over time. Right now it covers the interface, agents, memory, projects, shared folders, background tasks, system agents, access grants, connectors, skills, voice input and plugins; more sections (security groups…) will be added later.
 
 ## Features
 
 | Document | What it covers |
 | --- | --- |
 | [memory.md](memory.md) | Private and shared memory: what goes where, the indexes and history log, why some shared facts can't be changed on request |
+| [agents.md](agents.md) | Agents: the three kinds (chat, task, system), which one you are talking to and why, the specialist agents the assistant delegates to, how the model is chosen, and adding a custom agent |
 | [projects.md](projects.md) | Projects: shared folders with their own assistant chat, a live file explorer, and member sharing |
+| [shared-folders.md](shared-folders.md) | Shared folders: admin-managed folders with no chat of their own — who sees them, read vs write access, why the assistant asks before touching them, and when to choose a project instead |
 | [system-agents.md](system-agents.md) | Background agents that run on a schedule (event triage, the two memory lints, the nightly conversation review of a supervised account): what they watch, why they only ever report, why a run can be skipped, and their settings |
 | [tasks.md](tasks.md) | Background tasks: the strip above the message box, following one live, stopping one, answering the approvals and questions they raise, and how every outcome comes back to the conversation |
 | [settings.md](settings.md) | The admin's Config page: interface language, the compaction model picker, debug mode |
 | [access.md](access.md) | Who can use which plugin or connector: the open default, removing access per person, and the role switch that keeps children out of it |
 | [connectors.md](connectors.md) | Connectors (MCP servers): shared vs per-user, setting one up in the UI, the sign-in and QR-pairing flows, and what to do when one is not working |
+| [skills.md](skills.md) | Skills: instruction folders the assistant loads on demand — where they live, how to read and run one, and the contract for writing, installing and downloading one |
 | [voice.md](voice.md) | Voice input: configuring a transcription model, and why the microphone button does nothing unless the page is served over HTTPS or localhost |
 | [interface.md](interface.md) | The desktop interface: collapsing the sidebar to an icon-only strip to make room for documents |