feat(grok): price grok-build-0.1 + secret-scrub opencode plugin

- pricing.py: add grok-build-0.1 rates ($1/1M input, $0.20 cached, $2/1M
  output), verified against xAI's published pricing. Grok is a priced
  non-Anthropic model, so cost computes the moment usage is captured.
- secret-scrub.js: an opencode tool.execute.before plugin porting the
  security-critical bash-guard deny rules (git network ops, credential-file
  reads, /proc env, internal-host HTTP, roboco.* imports, ROBOCO_AGENT_ID
  forgery, env dumps, destructive rm) to the opencode runtime — restoring the
  guard the Claude Code hook can't provide there. Throwing denies the call
  (confirmed by opencode's env-protection example). Wired into the generated
  opencode.json plugin array + baked into the grok image.

Deny logic verified via node (9 deny + 5 allow cases). UNVALIDATED against a
live opencode runtime: confirm it fires in the live E2E spawn before a Grok
dev-agent touches a real repo; the bash permission is operator-tunable as a
second gate.

Cost CAPTURE (distinct from pricing) is intentionally NOT built yet: opencode's
plugin hooks expose model info but no token/usage object, so the capture path
is unconfirmed and needs the live spawn to settle.

tests/unit/llm/test_opencode_config.py

@@ -80,6 +80,8 @@ def test_build_opencode_config_provider_and_model() -> None:
     # Gateway servers carried through.
     assert "roboco-flow" in cfg["mcp"]
     assert cfg["instructions"] == ["/app/system-prompt.md"]
+    # The secret-scrub command guard is wired in by default.
+    assert cfg["plugin"] == ["/app/opencode-plugins/secret-scrub.js"]
 
 
 def test_build_opencode_config_bash_permission_is_tunable() -> None: