chore(infra): adjust main CI checks

Why:
- run main checks across OS matrix without building binaries
- keep release builds tied to tags

Impact:
- main pushes run just check on six OS targets with placeholders
- tag pushes still build and publish artifacts
- Tests: just check

Author: icepuma

.github/workflows/ci.yml

@@ -12,6 +12,7 @@ on:
 jobs:
   prebuild-agents:
     name: Prebuild Agents
+    if: github.event_name == 'pull_request' || startsWith(github.ref, 'refs/tags/v')
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout
@@ -121,9 +122,99 @@ jobs:
         shell: pwsh
         run: just check
 
+  check-main:
+    name: Check - main - ${{ matrix.platform.os-name }}
+    if: (github.event_name == 'push' && github.ref == 'refs/heads/main') || github.event_name == 'workflow_dispatch'
+    strategy:
+      fail-fast: false
+      matrix:
+        platform:
+          - os-name: Linux-x86_64
+            runs-on: ubuntu-24.04
+            target: x86_64-unknown-linux-musl
+          - os-name: Linux-aarch64
+            runs-on: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-musl
+          - os-name: macOS-x86_64
+            runs-on: macos-15-intel
+            target: x86_64-apple-darwin
+          - os-name: macOS-aarch64
+            runs-on: macos-26
+            target: aarch64-apple-darwin
+          - os-name: Windows-x86_64
+            runs-on: windows-2025
+            target: x86_64-pc-windows-msvc
+          - os-name: Windows-aarch64
+            runs-on: windows-11-arm
+            target: aarch64-pc-windows-msvc
+    runs-on: ${{ matrix.platform.runs-on }}
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Install Rust
+        uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt, clippy
+
+      - name: Add musl targets
+        run: rustup target add x86_64-unknown-linux-musl aarch64-unknown-linux-musl
+
+      - name: Install just (Unix)
+        if: runner.os != 'Windows'
+        uses: taiki-e/install-action@v2
+        with:
+          tool: just
+
+      - name: Install just (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: choco install just -y
+
+      - name: Install cargo-nextest
+        uses: taiki-e/install-action@v2
+        with:
+          tool: cargo-nextest
+
+      - name: Set placeholder agent paths (Unix)
+        if: runner.os != 'Windows'
+        run: |
+          echo "INTAR_AGENT_X86_64_PATH=$RUNNER_TEMP/intar-agent-x86_64" >> "$GITHUB_ENV"
+          echo "INTAR_AGENT_AARCH64_PATH=$RUNNER_TEMP/intar-agent-aarch64" >> "$GITHUB_ENV"
+
+      - name: Set placeholder agent paths (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          "INTAR_AGENT_X86_64_PATH=$env:RUNNER_TEMP\\intar-agent-x86_64" | Out-File -FilePath $env:GITHUB_ENV -Append -Encoding utf8
+          "INTAR_AGENT_AARCH64_PATH=$env:RUNNER_TEMP\\intar-agent-aarch64" | Out-File -FilePath $env:GITHUB_ENV -Append -Encoding utf8
+
+      - name: Create placeholder agents (Unix)
+        if: runner.os != 'Windows'
+        run: |
+          echo -n "PLACEHOLDER_AGENT_BINARY" > "$INTAR_AGENT_X86_64_PATH"
+          echo -n "PLACEHOLDER_AGENT_BINARY" > "$INTAR_AGENT_AARCH64_PATH"
+
+      - name: Create placeholder agents (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: |
+          "PLACEHOLDER_AGENT_BINARY" | Set-Content -NoNewline -Path $Env:INTAR_AGENT_X86_64_PATH
+          "PLACEHOLDER_AGENT_BINARY" | Set-Content -NoNewline -Path $Env:INTAR_AGENT_AARCH64_PATH
+
+      - name: Run just check (Unix)
+        if: runner.os != 'Windows'
+        run: just check
+
+      - name: Run just check (Windows)
+        if: runner.os == 'Windows'
+        shell: pwsh
+        run: just check
+        run: just check
+
   build:
     name: Build - ${{ matrix.platform.os-name }}
-    if: github.ref == 'refs/heads/main' || startsWith(github.ref, 'refs/tags/v')
+    if: startsWith(github.ref, 'refs/tags/v')
     needs: prebuild-agents
     env:
       INTAR_AGENT_X86_64_PATH: ${{ github.workspace }}/prebuilt-agents/intar-agent-x86_64