fix(grok): harden cost-guard, pin runtime, refresh stale plugin comments

Address review findings on the Grok provider work:

- budget-feed plugin failed open unconditionally, so a one-shot task agent
  whose in-container SDK budget server went unreachable would run with the
  cost cap unenforced. The entrypoint now exports ROBOCO_BUDGET_ENFORCE=1
  (one-shot agents always start that server) and the plugin's pre-exec gate
  fails CLOSED when the flag is set and the budget endpoint is unreachable,
  halting an uncapped burn. Interactive serve agents (intake/secretary) set
  no flag and keep failing open (they run no budget server by design).

- Pin opencode-ai to the live-verified 1.17.8 (was an unpinned global npm
  install). Untrusted model output runs under it; bump the pin deliberately.

- Document the ROBOCO_GROK_* operator vars in .env.example (image, the three
  opencode permissions, reasoning effort, idle-kill, cost ceiling).

- Refresh stale plugin comments: the MCP tool-name shape and the secretary
  tool-registration path are confirmed live, and secret-scrub's load route is
  the auto-discovery dir (not a config plugin: array). Keep the honest
  not-yet-exercised caveat on secret-scrub's deny path and the reasoning
  variant — those remain genuinely unverified.

docker/agent-grok.Dockerfile

@@ -15,7 +15,10 @@ USER root
 # opencode — the OpenAI-protocol agent runtime. grok-build-0.1 runs on opencode's
 # BUILT-IN xai provider (no custom provider block / npm needed), so only
 # opencode-ai is installed; it resolves the provider SDK at runtime.
-RUN npm install -g opencode-ai \
+# Pinned: this version is the live-verified runtime (grok-build-0.1 on the NAS).
+# Untrusted model output runs under it, so bump the pin deliberately, never float.
+ARG OPENCODE_VERSION=1.17.8
+RUN npm install -g "opencode-ai@${OPENCODE_VERSION}" \
     && npm cache clean --force \
     && rm -rf /root/.npm /tmp/*