feat: add Grok as a --guest runtime for Codex $cc commands

[CBEPX]

Summary

Add an opt-in Grok guest path to the Codex-hosted Claude Code companion so $cc:review, $cc:adversarial-review, $cc:rescue, and $cc:setup can run Grok instead of Claude Code.

• New --guest <claude|grok> flag (default claude). CC_GUEST is the env fallback; the CLI flag wins.
• Grok turns spawn grok --prompt-file … --output-format json --always-approve, not claude -p and not codex exec.
• Reviews keep the existing isolated git worktree, then restrict Grok to read_file,grep,list_dir,web_search,web_fetch.
• Claude remains the default path. Existing Claude model aliases, MCP git, sandbox settings, and job titles are unchanged unless --guest grok is set.
• --user-mcp-tool / --allow-project-mcp-servers stay Claude-only and error on --guest grok.
• Grok does not inherit Claude aliases such as opus. Omit --model to use Grok's default, or pass a Grok model ID.

Please review this PR with Codex and Claude. Do not merge until both reviews land.

Test plan

• npm test in the worktree: 792/792 pass (770 previous + 22 new guest/grok-cli/companion tests)
• npm run typecheck
• npm run lint
• Reviewers: hermetic fake-grok coverage is in tests/grok-cli.test.mjs; live $cc:review --guest grok --wait is intentionally not run here (paid API)

Hermetic coverage added:

• resolveGuest default / env / override / rejection
• buildGrokArgs uses --prompt-file + json, never Claude stream-json / --allowedTools
• runGrokTurn writes the prompt file and parses { text, sessionId, modelUsage }
• companion --help documents --guest
• review --guest chatgpt fails with Unsupported guest before git/Claude

[CBEPX]

Superseded by #36 — same Grok-guest feature, rebased onto v1.7.3. This branch was cut from v1.6.1 and is DIRTY against current main.

[CBEPX]

Closing in favor of #36.