fix(obsidivm): actionable error when the range service is unreachable (#156) (#158)

* test(obsidivm): RED — bridge should give an actionable error when the service is down

The obsidivm:* benchmarks fetch from the OBSIDIVM range service; when it is not
running the bridge throws a bare 'fetch failed' (surfaced as 'FATAL: fetch
failed'), naming neither the service, the URL, nor how to start it. Pins that a
connection failure becomes an actionable, self-service error.

Fails on main: the message is 'fetch failed'.

Refs #156

* fix(obsidivm): actionable error when the range service is unreachable

The obsidivm:* benchmarks fetch from the OBSIDIVM range service; when it is not
running the bridge's shared call() let the raw 'fetch failed' propagate, which
surfaced as an opaque 'FATAL: fetch failed' (#156) naming neither the service,
the URL, nor how to start it — even in --hunter stub, whose grading step still
needs it. Wrap the fetch in call() (the single choke point for every OBSIDIVM
request) so a connection failure becomes: 'OBSIDIVM service unreachable at <url>
... Start it per docs/OBSIDIVM.md (python3 range.py, listens on :4200), or set
OBSIDIVM_URL', flagged err.unreachable to distinguish it from an HTTP-status
error; a timeout is reported distinctly. Clarify in the bench --help/usage that
every hunter mode (stub included) needs the OBSIDIVM service, live/t3mp3st also
a key. Adds scripts/obsidivm-bridge.d.mts so the new test types cleanly.

Closes #156

* fix(obsidivm): distinguish invalid-URL from unreachable; test timeout path

Adversarial review of the diff found two gaps:
- A malformed OBSIDIVM_URL (ERR_INVALID_URL) was folded into the 'service
  unreachable — is it running?' path, misdirecting the user to start a service
  when the real fix is the URL. Now branched to a distinct 'Invalid OBSIDIVM
  base URL … set OBSIDIVM_URL' error, no err.unreachable flag.
- The timeout (AbortError) branch had no coverage. Added a test using a
  never-responding TCP server + short timeoutMs asserting 'timed out after Nms'
  and NOT unreachable; and a test for the invalid-URL branch.

Also completed the .d.mts stub with the cloudgoat* methods it was missing.

Refs #156

scripts/obsidivm-bench.mjs

@@ -7,7 +7,9 @@
  *   2. Dispatch a t3mp3st hunter against the target's URL.
  *      - --hunter=live   : direct LLM (OpenRouter / Anthropic / OpenAI auto-detect)
  *      - --hunter=t3mp3st: drive t3mp3st's /api/general/auto (full platform)
- *      - --hunter=stub   : synthesized transcript (no LLM, for plumbing tests)
+ *      - --hunter=stub   : synthesized transcript (no LLM, for plumbing tests).
+ *                          NB: only the HUNTER is offline — the OBSIDIVM service
+ *                          is still required for the spec (step 1) and scoring (3).
  *   3. Submit the agent transcript to OBSIDIVM's /api/score/text.
  *   4. Persist the run via OBSIDIVM's /api/runs ledger + per-target sessions.
  *   5. Print a per-target table + suite aggregate (weighted % + grade).
@@ -105,9 +107,12 @@ function help() {
 Options:
   --target <id>           Target id (repeatable). Default: all 14 OBSIDIVM targets.
   --hunter <stub|live|t3mp3st>
-                          stub:    synthesized transcript (no LLM)
+                          stub:    synthesized transcript (no LLM; the OBSIDIVM
+                                   service is still required for spec + scoring)
                           live:    direct LLM call
                           t3mp3st: drive t3mp3st's /api/general/auto
+  All modes need the OBSIDIVM service (see --obsidivm / docs/OBSIDIVM.md); live
+  and t3mp3st additionally need an LLM key.
   --obsidivm <url>        OBSIDIVM base URL (default http://127.0.0.1:4200)
   --t3mp3st <url>         t3mp3st base URL (default http://127.0.0.1:3333)
   --model <name>          LLM model id (default claude-opus-4-7)