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/scripts/grok-agent-entrypoint.sh

@@ -33,6 +33,12 @@ fi
 # Zero the budget/terminal counters at the start of the session.
 curl -sf -m 2 -X POST "${SDK_URL}/budget/reset" >/dev/null 2>&1 || true
 
+# This is a one-shot delivery agent: the SDK budget server above is mandatory.
+# Tell the budget-feed plugin to FAIL CLOSED if that server ever goes
+# unreachable mid-run, so an unenforceable cost cap halts the burn instead of
+# letting it run uncapped. (Interactive serve images set no such flag.)
+export ROBOCO_BUDGET_ENFORCE=1
+
 # Prompt-injection guard (parity with the Claude UserPromptSubmit hook): the
 # task prompt is DATA, not instructions — refuse a poisoned one before it
 # reaches the model. Same patterns as docker/scripts/user-prompt-hook.sh.