fix(ci/harness-replays): add fetch-depth:0 to detect-changes checkout

The detect-changes step runs `git diff "$base_sha" "$head_sha"` but the
preceding `actions/checkout` uses the default fetch-depth: 1 — only the
PR head commit is fetched. The base ref (github.event.pull_request.base.sha)
is not in the local history, so git diff fails silently (2>/dev/null),
leaving DIFF empty and the step exits non-zero. With continue-on-error: true
on the job, the step reports "failure" instead of blocking the PR, but the
output is never written so downstream harness-replays always skips.

Fix: add fetch-depth: 0 to the detect-changes checkout step so full history
is fetched and both base and head refs exist locally.

Spotted during gate triage review (core-lead-agent, 2026-05-11).

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

.gitea/workflows/harness-replays.yml

@@ -68,6 +68,17 @@ jobs:
       run: ${{ steps.decide.outputs.run }}
     steps:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+        with:
+          # fetch-depth: 0 is required here. The step runs `git diff "$base_sha"
+          # "$head_sha"` — if only the head commit is fetched (default
+          # fetch-depth: 1), the base ref is not in the local history and
+          # git diff fails silently (2>/dev/null), leaving DIFF empty, the
+          # step exits non-zero, and detect-changes reports "failure" despite
+          # continue-on-error: true. With fetch-depth: 0 the full history is
+          # available so both refs exist locally. The PR head and the base
+          # branch tip are always on the remote; full history is needed only
+          # to make them accessible offline in the container.
+          fetch-depth: 0
       - id: decide
         run: |
           # workflow_dispatch: always run (manual trigger)