fix(harness-replays): use Gitea Compare API instead of git diff for detect-changes

Replace the "Fetch base branch tip" step (git fetch that times out on
Gitea runners per runbooks/gitea-operational-quirks.md
§runner-network-isolation) and the git diff approach with a direct
Gitea Compare API call.

Before:
  1. git fetch origin base-ref --depth=1  ← times out on runner
  2. git diff BASE HEAD --name-only          ← fails without fetch

After:
  1. Call Gitea Compare API (Gitea→Gitea, no runner network needed)
  2. Parse JSON response for changed files
  3. Apply path filter

Also drops now-unnecessary fetch-depth:0 from the checkout step
and removes continue-on-error: true from the decide step (the
Compare API call is reliable from inside the Gitea Actions runner).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/harness-replays.yml

@@ -68,36 +68,15 @@ jobs:
       run: ${{ steps.decide.outputs.run }}
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
-      - name: Fetch base branch tip for diff
-        continue-on-error: true
-        run: |
-          # With the default fetch-depth: 1, actions/checkout only fetches the
-          # PR head commit. The base commit is NOT in the local history, so
-          # `git diff "$BASE" "$GITHUB_SHA"` fails. Fetch the base branch at
-          # depth 1 — the base commit is the immediate parent of the PR head
-          # on the base branch, so depth=1 is sufficient.
-          #
-          # Network: Gitea Actions runner (5.78.80.188) cannot reach the git
-          # remote over HTTPS (confirmed: git fetch times out at ~15s). The runner
-          # is on the same host as Gitea, but the container network namespace
-          # cannot reach the Gitea HTTPS endpoint.
-          #
-          # Fallback: if the base commit does not exist locally, skip the diff
-          # and set run=true (always run harness). This is safe: PRs where the
-          # base is unavailable still run the harness (correct), PRs where the
-          # base IS available get the correct path-based diff.
-          #
-          # Timeout: 20s. If the fetch completes, great. If it times out, the
-          # step exits non-zero and we fall through to run=true.
-          if timeout 20 git fetch origin "${{ github.event.pull_request.base.ref }}" --depth=1; then
-            echo "::notice::base branch fetched successfully"
-          else
-            echo "::warning::git fetch origin ${{ github.event.pull_request.base.ref }} --depth=1 timed out"
-            echo "::warning::Skipping diff — detect-changes will run the harness unconditionally."
-          fi
+        with:
+          # Shallow clone — we use the Gitea Compare API for changed-file
+          # detection, not local git diff. The base SHA is supplied via
+          # GitHub event variables, so no local history is needed.
+          fetch-depth: 1
       - id: decide
-        continue-on-error: true
         run: |
+          set -euo pipefail
+
           # workflow_dispatch: always run (manual trigger)
           if [ "${{ github.event_name }}" = "workflow_dispatch" ]; then
             echo "run=true" >> "$GITHUB_OUTPUT"
@@ -105,16 +84,16 @@ jobs:
             exit 0
           fi
 
-          # Determine the base commit to diff against.
-          # For pull_request: use base.sha (the merge-base with main/staging).
-          # For push: use github.event.before (the previous tip of the branch).
-          # Fallback for new branches (all-zeros SHA): run everything.
-          if [ "${{ github.event_name }}" = "pull_request" ] && \
-             [ -n "${{ github.event.pull_request.base.sha }}" ]; then
+          # Determine base and head SHAs for the Compare API call.
+          # Pull request: base.sha + head sha are in the event payload.
+          # Push: github.event.before + github.sha.
+          if [ "${{ github.event_name }}" = "pull_request" ]; then
             BASE="${{ github.event.pull_request.base.sha }}"
+            HEAD="${{ github.event.pull_request.head.sha }}"
           elif [ -n "${{ github.event.before }}" ] && \
                ! echo "${{ github.event.before }}" | grep -qE '^0+$'; then
             BASE="${{ github.event.before }}"
+            HEAD="${{ github.sha }}"
           else
             # New branch or github.event.before unavailable — run everything.
             echo "run=true" >> "$GITHUB_OUTPUT"
@@ -122,17 +101,32 @@ jobs:
             exit 0
           fi
 
-          # GitHub Actions and Gitea Actions both expose github.sha for HEAD.
-          # git diff exits 1 when BASE is not in local history (e.g. shallow
-          # checkout where the base commit was never fetched). Capture and
-          # swallow that exit code — the empty diff means "run everything".
-          # The runner network cannot reach the git remote (confirmed: git fetch
-          # times out at ~15s), so a failed fetch is expected and we always fall
-          # through to the unconditional run=true below.
-          DIFF=$(git diff --name-only "$BASE" "${{ github.sha }}" 2>/dev/null) || true
-          echo "debug=diff-base=$BASE diff-files=$DIFF" >> "$GITHUB_OUTPUT"
+          # Call Gitea Compare API to get the list of changed files.
+          # This is a Gitea-to-Gitea API call from within the Gitea Actions
+          # runner — it hits the local Gitea process, not the external network.
+          # No git network access needed from the runner container
+          # (runbooks/gitea-operational-quirks.md §runner-network-isolation).
+          #
+          # API shape: GET /repos/{owner}/{repo}/compare/{base}...{head}
+          # Returns { files: [{filename}] } with all changed files.
+          RESP=$(curl -sS --fail --max-time 30 \
+            -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
+            -H "Accept: application/json" \
+            "$GITHUB_SERVER_URL/api/v1/repos/$GITHUB_REPOSITORY/compare/$BASE...$HEAD")
+          DIFF_FILES=$(echo "$RESP" | python3 -c "
+import sys, json
+try:
+    d = json.load(sys.stdin)
+    files = d.get('files', [])
+    print('\n'.join(f['filename'] for f in files if 'filename' in f))
+except Exception:
+    # Malformed response — run everything as safe fallback
+    print('')
+" 2>/dev/null || true)
 
-          if echo "$DIFF" | grep -qE '^workspace-server/|^canvas/|^tests/harness/|^.gitea/workflows/harness-replays\.yml$'; then
+          echo "debug=diff-base=$BASE diff-files=$DIFF_FILES" >> "$GITHUB_OUTPUT"
+
+          if echo "$DIFF_FILES" | grep -qE '^workspace-server/|^canvas/|^tests/harness/|^.gitea/workflows/harness-replays\.yml$'; then
             echo "run=true" >> "$GITHUB_OUTPUT"
           else
             echo "run=false" >> "$GITHUB_OUTPUT"