fix(action): use REST API for GitHub asset downloads, enforce trusted GITEA_URL

Addresses gpt-review-bot findings on PR #121:

MAJOR #1: The 'Run review' step set GITEA_URL from inputs.gitea-url which
could exfiltrate the reviewer token to an attacker-controlled host on
GitHub/GHES. Now uses steps.version.outputs.server_url which enforces
VCS-type-aware trust (github.server_url on GitHub, validated input on Gitea).

MAJOR #2: Private release asset downloads on GitHub/GHES used web URLs
({server}/.../releases/download/{tag}/{asset}) which redirect to S3 and
don't support Authorization headers for private repos. Now uses the GitHub
REST API: fetches release metadata by tag, extracts asset IDs, and downloads
via /repos/{owner}/{repo}/releases/assets/{id} with Accept: octet-stream.
Gitea path retains direct URL downloads (which work correctly there).

.gitea/actions/review/action.yml

@@ -1,7 +1,7 @@
 # This composite action supports both Gitea Actions and GitHub Actions runners.
 # It detects the VCS host type using the github.api_url context (set only on
 # GitHub/GHES runners) and uses the appropriate releases API for version
-# resolution and binary download.
+# resolution and binary download (REST API on GitHub, direct URLs on Gitea).
 #
 # Security notes:
 # - On GitHub/GHES (VCS_TYPE=github), inputs.gitea-url is IGNORED to prevent
@@ -271,44 +271,93 @@ runs:
         ACTION_TOKEN="${ACTION_TOKEN:-}"
         BINARY="review-bot-linux-amd64"
 
-        # SECURITY: On GitHub/GHES, use github.server_url for downloads.
-        # SERVER_URL is already set correctly per VCS_TYPE in Determine version step,
-        # but verify the download destination matches expectations.
         if [ "$VCS_TYPE" = "github" ]; then
-          # Double-check: SERVER_URL must be github.server_url (platform-provided)
-          EXPECTED_SERVER="${{ github.server_url }}"
-          EXPECTED_SERVER="${EXPECTED_SERVER%/}"
-          if [ "$SERVER_URL" != "$EXPECTED_SERVER" ]; then
-            echo "Error: SERVER_URL mismatch on GitHub runner (possible tampering)" >&2
+          # GitHub/GHES: Use REST API for release asset downloads.
+          # Web release URLs ({server}/.../releases/download/{tag}/{asset}) redirect
+          # to S3 and don't reliably support Authorization headers for private repos.
+          # The REST API endpoint with Accept: application/octet-stream is required.
+          GITHUB_API_URL="${{ github.api_url }}"
+
+          if [ -n "$ACTION_TOKEN" ]; then
+            RELEASE_JSON=$(curl -sSf --connect-timeout 10 --max-time 30 \
+              -H "Authorization: Bearer ${ACTION_TOKEN}" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/tags/${VERSION}")
+          else
+            RELEASE_JSON=$(curl -sSf --connect-timeout 10 --max-time 30 \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/tags/${VERSION}")
+          fi
+
+          # Extract asset IDs for binary and checksums
+          BINARY_ASSET_ID=$(printf '%s' "$RELEASE_JSON" | python3 -c "
+import sys, json
+assets = json.load(sys.stdin).get('assets', [])
+for a in assets:
+    if a['name'] == '${BINARY}':
+        print(a['id'])
+        break
+else:
+    sys.exit(1)
+")
+          if [ -z "$BINARY_ASSET_ID" ]; then
+            echo "Error: could not find asset '${BINARY}' in release ${VERSION}" >&2
             exit 1
           fi
-        fi
 
-        # Download URL format is the same on both Gitea and GitHub:
-        # {server}/{owner}/{repo}/releases/download/{tag}/{asset}
-        DOWNLOAD_URL="${SERVER_URL}/${ACTION_REPO}/releases/download/${VERSION}"
+          CHECKSUMS_ASSET_ID=$(printf '%s' "$RELEASE_JSON" | python3 -c "
+import sys, json
+assets = json.load(sys.stdin).get('assets', [])
+for a in assets:
+    if a['name'] == 'checksums.txt':
+        print(a['id'])
+        break
+else:
+    sys.exit(1)
+")
+          if [ -z "$CHECKSUMS_ASSET_ID" ]; then
+            echo "Error: could not find asset 'checksums.txt' in release ${VERSION}" >&2
+            exit 1
+          fi
 
-        if [ -n "$ACTION_TOKEN" ]; then
-          if [ "$VCS_TYPE" = "github" ]; then
+          # Download assets via REST API with Accept: application/octet-stream
+          if [ -n "$ACTION_TOKEN" ]; then
             curl -sSfL --connect-timeout 10 --max-time 120 \
               -H "Authorization: Bearer ${ACTION_TOKEN}" \
-              "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${BINARY_ASSET_ID}" \
+              -o "${{ runner.temp }}/review-bot"
             curl -sSfL --connect-timeout 10 --max-time 30 \
               -H "Authorization: Bearer ${ACTION_TOKEN}" \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${CHECKSUMS_ASSET_ID}" \
+              -o "${{ runner.temp }}/checksums.txt"
+          else
+            curl -sSfL --connect-timeout 10 --max-time 120 \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${BINARY_ASSET_ID}" \
+              -o "${{ runner.temp }}/review-bot"
+            curl -sSfL --connect-timeout 10 --max-time 30 \
+              -H "Accept: application/octet-stream" \
+              "${GITHUB_API_URL}/repos/${ACTION_REPO}/releases/assets/${CHECKSUMS_ASSET_ID}" \
+              -o "${{ runner.temp }}/checksums.txt"
+          fi
+        else
+          # Gitea: Direct download via web release URLs (Gitea serves assets
+          # directly and supports token auth on these URLs)
+          DOWNLOAD_URL="${SERVER_URL}/${ACTION_REPO}/releases/download/${VERSION}"
+
+          if [ -n "$ACTION_TOKEN" ]; then
+            curl -sSfL --connect-timeout 10 --max-time 120 \
+              -H "Authorization: token ${ACTION_TOKEN}" \
+              "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
+            curl -sSfL --connect-timeout 10 --max-time 30 \
+              -H "Authorization: token ${ACTION_TOKEN}" \
               "${DOWNLOAD_URL}/checksums.txt" -o "${{ runner.temp }}/checksums.txt"
           else
             curl -sSfL --connect-timeout 10 --max-time 120 \
-              -H "Authorization: token ${ACTION_TOKEN}" \
               "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
             curl -sSfL --connect-timeout 10 --max-time 30 \
-              -H "Authorization: token ${ACTION_TOKEN}" \
               "${DOWNLOAD_URL}/checksums.txt" -o "${{ runner.temp }}/checksums.txt"
           fi
-        else
-          curl -sSfL --connect-timeout 10 --max-time 120 \
-            "${DOWNLOAD_URL}/${BINARY}" -o "${{ runner.temp }}/review-bot"
-          curl -sSfL --connect-timeout 10 --max-time 30 \
-            "${DOWNLOAD_URL}/checksums.txt" -o "${{ runner.temp }}/checksums.txt"
         fi
 
         # Verify SHA-256 checksum
@@ -336,7 +385,7 @@ runs:
     - name: Run review
       shell: bash
       env:
-        GITEA_URL: ${{ inputs.gitea-url || github.server_url }}
+        GITEA_URL: ${{ steps.version.outputs.server_url }}
         GITEA_REPO: ${{ inputs.repo || github.repository }}
         PR_NUMBER: ${{ inputs.pr-number || github.event.pull_request.number }}
         REVIEWER_TOKEN: ${{ inputs.reviewer-token }}