feat(action): derive binary name from uname for multi-arch support (#124)

Previously the Install step hard-coded 'review-bot-linux-amd64'. This
fails on arm64 runners (Graviton, Apple Silicon) where uname -m returns
'aarch64' or 'arm64'.

Changes:
- Add OS/arch detection in 'Determine version' step using uname -s/-m
- Map uname output to asset name format: x86_64→amd64, aarch64/arm64→arm64,
  linux→linux, darwin→darwin
- Emit 'os' and 'arch' as step outputs alongside 'version'
- Update cache key: review-bot-{os}-{arch}-{version}
- Update Install step: BINARY derived from step outputs
- Anchor checksum grep to exact filename (not substring match)
- Unsupported OS or arch exits with a clear error message

Supported platforms: linux-amd64, linux-arm64, darwin-amd64, darwin-arm64
(matches what the release workflow builds)

.gitea/actions/review/action.yml

@@ -124,14 +124,38 @@ runs:
         else
           VERSION="${{ inputs.version }}"
         fi
+
+        # Detect OS and architecture for platform-specific binary download
+        OS_RAW=$(uname -s | tr '[:upper:]' '[:lower:]')
+        case "$OS_RAW" in
+          linux)  OS="linux" ;;
+          darwin) OS="darwin" ;;
+          *)
+            echo "Error: unsupported OS: $(uname -s)" >&2
+            exit 1
+            ;;
+        esac
+
+        RAW_ARCH=$(uname -m)
+        case "$RAW_ARCH" in
+          x86_64)           ARCH="amd64" ;;
+          aarch64 | arm64)  ARCH="arm64" ;;
+          *)
+            echo "Error: unsupported architecture: $RAW_ARCH" >&2
+            exit 1
+            ;;
+        esac
+
         echo "version=${VERSION}" >> "$GITHUB_OUTPUT"
+        echo "os=${OS}" >> "$GITHUB_OUTPUT"
+        echo "arch=${ARCH}" >> "$GITHUB_OUTPUT"
 
     - name: Cache review-bot binary
       id: cache
       uses: actions/cache@v4
       with:
         path: ${{ runner.temp }}/review-bot
-        key: review-bot-linux-amd64-${{ steps.version.outputs.version }}
+        key: review-bot-${{ steps.version.outputs.os }}-${{ steps.version.outputs.arch }}-${{ steps.version.outputs.version }}
 
     - name: Install review-bot
       if: steps.cache.outputs.cache-hit != 'true'
@@ -140,7 +164,7 @@ runs:
         GITEA_URL="${{ inputs.gitea-url || github.server_url }}"
         REPO="${{ inputs.repo || 'rodin/review-bot' }}"
         VERSION="${{ steps.version.outputs.version }}"
-        BINARY="review-bot-linux-amd64"
+        BINARY="review-bot-${{ steps.version.outputs.os }}-${{ steps.version.outputs.arch }}"
 
         curl -sSfL "${GITEA_URL}/${REPO}/releases/download/${VERSION}/${BINARY}" \
           -o "${{ runner.temp }}/review-bot"
@@ -149,7 +173,7 @@ runs:
 
         # Verify SHA-256 checksum
         cd "${{ runner.temp }}"
-        EXPECTED=$(grep "${BINARY}" checksums.txt | awk '{print $1}')
+        EXPECTED=$(grep -E "[[:xdigit:]]+[[:space:]]+\*?${BINARY}$" checksums.txt | awk '{print $1}')
         ACTUAL=$(sha256sum review-bot | awk '{print $1}')
 
         if [ -z "$EXPECTED" ]; then
@@ -164,7 +188,7 @@ runs:
         fi
 
         chmod +x "${{ runner.temp }}/review-bot"
-        echo "Installed review-bot ${VERSION} (checksum verified)"
+        echo "Installed review-bot-${{ steps.version.outputs.os }}-${{ steps.version.outputs.arch }} ${VERSION} (checksum verified)"
 
     - name: Run review
       shell: bash