Add end-to-end test coverage for unch codex install

[uchebnick]

Problem

unch codex install is the main user-facing path for Codex integration. It currently needs stronger coverage around the actual files/config it writes and the assumptions Codex relies on afterward.

What to test

• Registers the unch MCP server in Codex config.
• Writes ~/.codex/skills/unch/SKILL.md.
• Removes legacy ~/.codex/prompts/unch.md if present.
• Preserves existing unrelated Codex config.
• Sets/keeps reasonable MCP startup and tool timeouts.
• Generated skill text instructs the model to pass the active repository directory.

Acceptance criteria

• Add a deterministic test for unch codex install using a temporary Codex home.
• Cover idempotent reruns.
• Cover dry-run output.
• Keep the test independent of a real user Codex config.