Add LLM-optimized --help output

[JohanLorenzo]

Summary

• When running inside a Claude Code session or other LLM environments (detected via CLAUDECODE, CODEX_SANDBOX, GEMINI_CLI, OPENCODE), --help prints a compact 16-line reference instead of the standard clap output.
• Follows the pattern from searchfox-cli.

Merge order

1. Fix git subprocess isolation in init #12 (fix-init-git-isolation) — no dependency
2. This PR — depends on Fix git subprocess isolation in init #12

Verification

CLAUDECODE=1 stmo-cli --help outputs the compact reference. Standard stmo-cli --help (without env var) shows normal clap output unchanged. All 66 tests pass.