fix(agent-gui): load capabilities for dollar skill queries

[7155]

Summary

• load provider capabilities when a Composer query starts with either / or $
• reuse the shared trigger-query parser so lazy loading follows the same trigger contract as the palette
• add a regression test for slash queries, dollar Skill queries, and ordinary text

Reproduction

1. Start Tutti with a fresh/cold AgentGUI state and select an Agent that exposes
   Skills.
2. In a new Composer, type $ before opening any capability menu or typing /.
3. The Skill palette is empty because no capability request is started.
4. Clear the draft, type /, and wait for capabilities to load.
5. Clear the draft and type $ again; the Skill palette now appears from the
   warmed catalog.

Verification

• pnpm check:changed -- --base upstream/main --push-ready — passed 12 lanes on the fix rebased over upstream/main@6d4778dc4
• regression test against unmodified upstream/main@6d4778dc4 — / and ordinary text passed; $ failed because the capabilities callback was called 0 times
• manual baseline A/B in a fresh Tutti Dev state:
  • cold $ showed no Skill palette
  • entering / loaded provider capabilities
  • entering $ afterward showed the Skill palette
• manual fixed-build verification in a second fresh state:
  • cold $ immediately showed the Skill palette without first entering /
  • the daemon completed the skills_only capability request with 10 options

Platform impact

• platform-neutral AgentGUI trigger parsing only
• no filesystem, process, shell, path, environment, or native-runtime behavior changed
• the regression runs in the package's JSDOM Vitest lane and applies equally to Windows and POSIX desktop renderers

Documentation

No durable documentation update is needed. This restores the existing documented $ Skill-palette contract and does not add a new user-facing feature, setup step, API, or runtime override.

Checklist

• This PR does not change session, turn, goal, or runtime-operation lifecycle semantics.
• I kept the change focused on one concern.
• Documentation impact was assessed; no update is required for this contract restoration.
• No README or CONTRIBUTING language variant is affected.
• I ran the lowest meaningful local checks for the changed surface.
• The commit is signed off with DCO.