feat: implement desktop detectors for Cursor and Codex App (issue #20)

[thundermiracle]

Summary

• extend config probe outcomes to distinguish:
  • override missing file
  • override permission denied
  • fallback permission denied
• surface desktop permission/access failures explicitly as error detection state
• keep deterministic path precedence (override first, fallback after)
• preserve desktop evidence model and candidate ordering for Cursor/Codex App
• add Rust tests for permission and missing-file scenarios
• add Node contract tests for desktop detector path candidates and permission signaling

Commit Structure

• feat(rust): add desktop detector error signaling for missing and permission failures (issue #20)
• test(ts): add desktop detector contracts and update cli reason-code assertions (issue #20)

Validation

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• pnpm outdated

Closes #20