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.

agents/README.md

@@ -1,3 +1,35 @@
+# Agents
+
+## Adding a new agent: the skills index is opt-in
+
+An agent sees the installed skills **only** if its `AGENT.md` carries the
+`<!-- SKILLS_LIST -->` placeholder, normally through
+`<!-- INCLUDE: common/skills.md -->`. There is no `meta.json` flag: the sentinel
+*is* the switch, exactly as it is for `<!-- MCP_LIST -->`.
+
+So a new agent starts **without** the index and stays without it until someone
+adds the line. That is the deliberate direction of the default: the opposite one
+— an agent inheriting the index by forgetfulness — is the worse failure, because
+the index is written in the imperative ("you MUST read its SKILL.md") and an
+unattended `type: system` agent has its approvals auto-denied and sometimes no
+tools at all.
+
+`common/skills.md` is **one line and deliberately holds no prose**, unlike
+`common/mcp.md`. Every word — the imperative header, the list, the closing rules
+— is produced by the renderer, so that an instance with no skills installed gets
+an empty string instead of a header promising a list that isn't there. (That is
+not hypothetical: the MCP section keeps its prose in the fragment, and its empty
+state once had the model invent a discovery tool to fill the gap.) The fragment
+cannot explain itself in place either — `resolve_includes` copies any line that
+is not an upper-case sentinel straight into the prompt, so a comment there would
+be read by the model.
+
+The rule of thumb: a `chat` or `task` agent gets the include, a `system` agent
+does not. Put the line **as low as possible** in the prompt (by convention right
+after `common/mcp.md`) — anything above it survives in the provider's cached
+prefix when a skill is added or removed. `crates/skald-core/src/agents.rs` has a
+test that holds every shipped agent to this.
+
 # Agent icons — style guide
 
 Each agent in the `agents/` directory can have an icon/avatar declared in the `"icon"` field of its `meta.json`. The backend serves the file via `GET /api/agents/{id}/icon`.