feat: add a Linux CI job alongside macOS, in one shared workflow

Renamed macos.yml to ci.yml with two jobs (macos, linux) under one
push/PR/dispatch trigger, so both run from the same commit instead of
needing separate files. Cache keys are now OS-prefixed (cargo-macos-/
cargo-linux-): target/ artifacts are platform-specific and would
otherwise collide under a shared key.

.forgejo/workflows/ci.yml

@@ -1,4 +1,4 @@
-name: macOS
+name: CI
 
 on:
   push:
@@ -7,7 +7,7 @@ on:
   workflow_dispatch:
 
 jobs:
-  check:
+  macos:
     runs-on: macos-arm64
     steps:
       - uses: https://code.forgejo.org/actions/checkout@v4
@@ -21,14 +21,17 @@ jobs:
           repository: randogoth/fumi
           ref: main
           path: fumi
+      # Cache key includes the OS explicitly: target/ artifacts are
+      # platform-specific, so macOS and Linux must never share a cache key
+      # or they'd clobber each other's compiled objects.
       - uses: https://code.forgejo.org/actions/cache@v4
         with:
           path: |
             ~/.cargo/registry
             carrier/target
             fumi/target
-          key: cargo-${{ hashFiles('carrier/Cargo.lock') }}
+          key: cargo-macos-${{ hashFiles('carrier/Cargo.lock') }}
-          restore-keys: cargo-
+          restore-keys: cargo-macos-
       # Forgejo auto-injects GITHUB_TOKEN (GH Actions compatibility), scoped
       # to this Forgejo instance. Nix auto-detects that env var and assumes
       # it's a github.com credential, so it sends it to api.github.com when
@@ -39,3 +42,27 @@ jobs:
         working-directory: carrier
         env:
           GITHUB_TOKEN: ""
+
+  linux:
+    runs-on: linux-x86_64
+    steps:
+      - uses: https://code.forgejo.org/actions/checkout@v4
+        with:
+          path: carrier
+      - uses: https://code.forgejo.org/actions/checkout@v4
+        with:
+          repository: randogoth/fumi
+          ref: main
+          path: fumi
+      - uses: https://code.forgejo.org/actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/registry
+            carrier/target
+            fumi/target
+          key: cargo-linux-${{ hashFiles('carrier/Cargo.lock') }}
+          restore-keys: cargo-linux-
+      - run: devbox run check
+        working-directory: carrier
+        env:
+          GITHUB_TOKEN: ""