ci: release workflow drafting a GitHub Release, and docs/releasing.md

A push to the release branch (or workflow_dispatch) builds the macOS universal dmg and CLI zip,
the Linux tar.gz and AppImage and the Windows zip, with the version from Cargo.toml. The last
job checks the set, writes SHA256SUMS.txt and notes saying how the macOS app was signed, and
creates or updates a draft release; it never publishes. Off the release branch it only uploads
the files as a run artifact (dry run). macOS signing and notarization use the APPLE_* secrets
when they exist and fall back to ad-hoc signing (#72).

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

assets/app-icon/README.md

@@ -22,6 +22,8 @@ hand. It runs on macOS (sips, iconutil, `xcrun actool`) with ImageMagick (`magic
 | `Assets.car` | `actool` compilation of `FieldCraft.icon` for macOS 26 and later (needs Xcode 26 or later; the task skips it with a note otherwise) |
 
 The desktop app embeds `icon.iconset/[email protected]` as its window and Dock icon
-(`apps/fieldcraft/src/main.rs`). There is no macOS bundle yet; packaging will copy
-`Assets.car` and `icon.icns` into `Contents/Resources` and name `FieldCraft` in
-`CFBundleIconName` / `CFBundleIconFile`.
+(`apps/fieldcraft/src/main.rs`). The release packages (docs/releasing.md) use the rest:
+`packaging/macos/package.sh` copies `Assets.car` and `icon.icns` (as `FieldCraft.icns`) into
+`FieldCraft.app/Contents/Resources`, named by `CFBundleIconName` / `CFBundleIconFile`;
+`apps/fieldcraft/build.rs` embeds `icon.ico` in `fieldcraft.exe`; and
+`packaging/linux/package.sh` installs the iconset PNGs as the hicolor icons.