chore: rename runner env var to PERMIT_AGENT_CLI

[FetchDoge]

• The LLM runner path is now read from PERMIT_AGENT_CLI. The old name PERMIT_GROK_CLI is still read as a deprecated fallback when PERMIT_AGENT_CLI is unset, so existing setups keep working.
• The two setup RuntimeError messages now say 'agent runner'.
• README, the submission checklist, the fixtures README and the demo hint use the new name. The README notes the deprecated alias once.
• Tests are updated to the new name, plus two new tests: the alias fallback works, and the new name takes precedence.

Local: 391 passed, 1 skipped (also under python -O).

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	⚠️ Failed 2026-10-15T21:34:27.193495Z	1705a8b	PR opened

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