feat: implement shared detector framework and evidence schema (issue #18)

[thundermiracle]

Summary

• add a dedicated detection layer with a shared ClientDetector interface and DetectorRegistry
• implement path-based detector framework with explicit status + confidence + evidence output
• add per-client detectors for Claude Code, Codex CLI, Cursor, and Codex App using one schema
• extend detection contract with confidence (0-100) while preserving status and evidence fields
• route detect_clients through detector registry while keeping list/mutate on adapter registry
• keep command boundary thin and UI-ready with no client-specific branching
• add Rust and Node tests covering schema consistency and partial-detection behavior

Commit Structure

• feat(rust): add shared client detector framework and evidence schema (issue #18)
• test(docs): align frontend contracts and boundary docs with detector schema (issue #18)

Validation

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• pnpm outdated

Closes #18