docs: add copilot instructions for future sessions

Document build/test/lint commands, the cmd -> download -> deezer
pipeline architecture, and codebase conventions so future Copilot
sessions can work effectively in this repository.

.github/copilot-instructions.md

@@ -0,0 +1,87 @@
+# GoDeez
+
+GoDeez is a Go CLI that downloads music (tracks, albums, playlists, artist top
+tracks) from Deezer in MP3 or lossless FLAC/WAV, embeds metadata tags, and
+tracks what has already been downloaded so repeat runs can skip it.
+
+## Build, test, lint
+
+Mirror what CI (`.github/workflows/ci.yml`) runs:
+
+```bash
+gofmt -l .        # must print nothing; CI fails on unformatted files
+go vet ./...
+go test ./...
+go build ./...
+```
+
+Run a single test:
+
+```bash
+go test ./internal/download/ -run TestHashFile
+go test ./internal/download/ -run TestHashFile -v
+```
+
+Release packaging is validated with `goreleaser check` (config in
+`.goreleaser.yaml`); releases are cut by pushing a `v*` tag, which requires a
+matching section in `CHANGELOG.md` (`.github/scripts/release-notes.sh`
+extracts it and fails the release otherwise).
+
+## Architecture
+
+The flow is `cmd` → `download.Downloader` → `deezer` client → `store`/`tag`/`audio`:
+
+- **`cmd`** is a thin cobra layer (`download`, `login`, `logout`, `update`,
+  `version`). It parses flags and delegates; it does not contain download
+  logic. `cmd.Execute` also kicks off an async update check
+  (`internal/update.StartCheck`) before running the command and prints the
+  result after, so the network round trip overlaps with the command instead
+  of adding to startup time.
+- **`internal/deezer`** is the API client. `Kind` (`album`/`playlist`/
+  `artist`/`track`) is the single source of truth mapping a resource type to
+  its gw-light page method, request id field, and `Resource` implementation
+  (`Album`/`Playlist`/`Artist`/`Single`). Adding a new resource kind means
+  extending every switch in `kind.go`. `Resource`'s `decode` method is
+  unexported to seal the interface to this package.
+- **`internal/download`** (package `download`) drives the per-track pipeline
+  via `Downloader.Run`: fetch resource → for each track, resolve a media
+  source → decide skip/hash-dedupe → stream and decrypt → optionally convert
+  FLAC to WAV → write tags → record in the store. Tracks are processed
+  sequentially, one at a time. Most per-track failures (bad cover, failed BPM
+  lookup, etc.) are collected as warnings rather than aborting the whole run;
+  only `context.Canceled` stops the loop early.
+- **`internal/store`** is a bbolt-backed ledger (`.tracks.db`) written as a
+  hidden file inside the output directory, so it travels with the music
+  library it describes. Losing it is harmless — the only cost is
+  re-downloading.
+- **`internal/tag`** writes metadata per container format (ID3v2 for MP3,
+  Vorbis comments for FLAC, ID3 chunk + RIFF LIST/INFO for WAV), dispatching
+  on file extension. Taggers write through a temp file so a mid-write failure
+  can't corrupt existing audio.
+- **`internal/config`** has no config file — the output directory
+  (`~/Music/GoDeez`) is fixed, and the only setting is the `DEEZER_ARL`
+  env var, which is optional (falls back to keyring-stored credentials from
+  `godeez login`).
+- **`internal/buildinfo`** holds the version/commit/date injected by
+  goreleaser via `-ldflags`; unreleased builds report `dev`, which disables
+  update-related behavior (`IsDev()`).
+- **`internal/update`** implements self-update: checks GitHub releases,
+  verifies the downloaded binary's SHA256 against the published
+  `checksums.txt`, and replaces the running binary in place.
+
+## Conventions
+
+- Package- and function-level doc comments consistently explain *why*, not
+  just what — e.g. why a check happens where it does, or why an error is
+  handled a particular way. Preserve this style when touching existing code
+  or adding new exported functions.
+- Most per-track errors during a download are non-fatal and surfaced as
+  warnings in the run summary; only cancellation (`context.Canceled`) should
+  abort the loop. Keep new failure modes inside this pattern unless they are
+  truly unrecoverable for the whole run.
+- `Kind` in `internal/deezer/kind.go` is deliberately exhaustive with no
+  default case that silently returns a zero value in the id/method switches —
+  follow the same pattern (explicit cases, empty/error fallback) if extending
+  it.
+- Credentials: `DEEZER_ARL` always takes precedence over keyring-stored
+  login credentials when both are present.