feat: implement cli detectors for Claude Code and Codex CLI (issue #19)

[thundermiracle]

Summary

• harden CLI detection reasoning with explicit reason codes:
  • detected
  • config_missing
  • binary_missing
  • binary_and_config_missing
  • config_override_invalid
• enforce deterministic precedence: explicit override path is evaluated before fallback paths
• add probe helper to test precedence behavior without runtime env mutation
• add fixture-style Rust tests for CLI detection state transitions and path precedence
• add Node contract tests for Claude/Codex CLI candidate ordering and reason-code presence

Commit Structure

• feat(rust): harden cli detector status reasoning and precedence handling (issue #19)
• test(ts): add cli detector contract tests for candidate order and reason codes (issue #19)

Validation

• pnpm run lint
• pnpm test
• cargo test --manifest-path src-tauri/Cargo.toml
• pnpm outdated

Closes #19