docs: add CLAUDE.md

Guidance for Claude Code sessions: the -tags webkit2_41 test
requirement, the sqlc/templ codegen workflow, the golangci-lint v2
rules, the conventional-commits requirement, and a sketch of the
Wails app lifecycle + backend package responsibilities.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -0,0 +1,75 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project
+
+YellowJacket is a cross-platform desktop music player built with Go (backend) and TypeScript/Lit (frontend), using the Wails framework to bridge them. It supports MP3, FLAC, OGG Vorbis, and WAV playback.
+
+## Commands
+
+```bash
+make dev              # Hot-reload development (installs deps, generates code, cleans frontend)
+make dev-debug        # Same as dev but with YJ_LOG_LEVEL=debug
+make build-dev        # Debug build with symbols
+make build-prod       # Production build (stripped, UPX-compressed)
+make generate         # Run code generators (sqlc + templ via go generate)
+make lint             # golangci-lint v2 (strict)
+make test             # All tests with race detector, 2min timeout
+make vulncheck        # govulncheck for CVEs
+make setup            # Install go tools, frontend deps, git hooks (lefthook)
+```
+
+### Running tests
+
+All Go test commands require the `-tags webkit2_41` build tag:
+
+```bash
+go test -tags webkit2_41 ./...                           # All tests
+go test -tags webkit2_41 ./backend/player/               # Single package
+go test -tags webkit2_41 -run TestName ./backend/player/  # Single test
+```
+
+Audio playback integration tests require `YELLOWJACKET_INTEGRATION=1`.
+
+## Architecture
+
+**Wails app lifecycle** (`main.go` → `backend/app.go`): `YellowJacketApp` is the root struct bound to Wails. Its methods are callable from the frontend. Lifecycle hooks: `OnStartup` (init audio), `OnDomReady` (start library scan), `OnBeforeClose` (save window state), `OnShutdown` (persist player/queue state).
+
+**Backend packages** (under `backend/`):
+- `player` — Audio playback via beep. `BufferedStreamer` provides a ring buffer for smooth seeking.
+- `queue` — Track queue with shuffle (Fisher-Yates), repeat modes, auto-advance, and session persistence.
+- `library` — Concurrent library scanning, metadata extraction, cover art deduplication, incremental rescan.
+- `database` — SQLite via pure-Go driver. Schema in `database/sql/schemas/`, queries in `database/sql/queries/`. **sqlc** generates Go code into `database/sql/sqlcgen/` — never edit that directory by hand.
+- `metadata` — Tag extraction (ID3v2, Vorbis Comments, FLAC).
+- `config` — TOML-based settings. Settings page uses HTMX + templ for server-rendered HTML fragments.
+- `playlist` / `smartplaylist` — Playlist CRUD and rule-based smart playlists.
+- `mediacontrols` — MPRIS integration on Linux via D-Bus.
+- `system` — OS-specific paths (XDG on Linux, `%LOCALAPPDATA%` on Windows).
+- `profiling` — pprof server on `:6060`, compiled out in non-dev builds via build tags (`internal/dev/`).
+
+**Frontend** (`frontend/`): Lit 3.2 web components + Web Awesome UI library + HTMX. State management via singleton reactive stores in `src/store/`. Wails bindings auto-generated in `frontend/wailsjs/` — don't edit by hand.
+
+**Event-driven communication**: Backend emits events via Wails runtime; frontend stores subscribe to them. Event names are constants in `backend/events/`.
+
+## Code Generation
+
+Two generators run via `go generate ./...` (or `make generate`):
+1. **sqlc** — SQL → Go. Config at `backend/database/sqlc.yaml`. Add queries in `backend/database/sql/queries/`, get generated Go in `database/sql/sqlcgen/`.
+2. **templ** — `.templ` files → `*_templ.go` files (same directory).
+
+Pre-commit hooks verify generated code is fresh — always run `make generate` after changing `.sql` or `.templ` files.
+
+## Code Style
+
+- **Go**: golangci-lint v2 with strict linters including `err113` (static errors), `nlreturn`, `wsl_v5` (whitespace), `godot` (comment periods), `sloglint`, `perfsprint`. Imports grouped: stdlib → third-party → `yellowjacket/...` (enforced by gci).
+- **TypeScript**: Strict mode, no implicit any, no unused locals/parameters.
+- **Commits**: Conventional commits format (enforced by commitlint in CI). Semantic release uses these for versioning.
+
+## Testing
+
+Tests use `database.NewTestDB(t)` for in-memory SQLite with full schema. Test audio fixtures live in `test_data/music_library_test/`. Table-driven tests are the norm.
+
+## Git Workflow
+
+Direct push to `main` is blocked by lefthook — use feature branches and PRs. Pre-commit runs vet, lint, codegen check, and frontend typecheck in parallel. Pre-push runs the full test suite.