fix: llama-down failures answer fast and say what to do

[plombeer31]

What

With llama-server down, the most common first-run failure was 17 seconds of dead air
followed by the two words fetch failed — no URL, no diagnosis, no fix. After this change
the same run fails in ~2 seconds and says:

! [transport] fetch failed
    llama-server is not reachable at http://127.0.0.1:8080
      start it with:       atomic-agent models start
      or point elsewhere:  atomic-agent config set localModels.url <url>

Where the 17 seconds went

createAgentRuntime runs checkLlamaServer() with the default retry ladder — 5 attempts,
exponential backoff, 500·(1+2+4+8+16) = 15.5 s — purely to log a warning
(bootstrap.ts, the probe never throws). The sidecar's start_session status probe had
the same ladder. Both now probe once: these are observability probes, and with a
refused connection the retries change nothing — the first real completion is the retry.
The TUI startup gate already probed with retries: 0; now every probe agrees.

The message

New formatLlamaUnreachableHint(url) in llama-server-health.ts — the shared, actionable
wording. run appends it to the first transport failure (step_error / loop_failed)
only when the active text route is the local server — on a cloud route the same
category points at the provider, not llama — and only once per process, so retries
don't bury the log in repeated advice.

Testing

• 6 new unit tests: hint content; appended on local-route transport failures; printed
  once, not per retry; untouched on cloud routes; untouched for non-transport categories;
  loop_failed decorated the same way. Plus the existing health-check suite (15/15 total).
• End-to-end against the built CLI, llama down, fresh state dir:
  before 17.3 s + bare fetch failed → after 2 s, exit 1, hint printed with URL and
  both fix commands.
• npm run lint clean.

Not covered here: the TUI feed still prints the bare ! [transport] fetch failed line —
that surface already has the health pill and the startup wizard, and enriching it touches
the reducer path, so it is left for a separate change.