fix(harness-replays): use github.sha not branch-name as HEAD for push events

Root cause: PR #497 used HEAD="${GITHUB_REF#refs/heads/}" (branch name) for
push events. Gitea Compare API returns empty divergent-commits when comparing
a SHA (github.event.before) with a branch name (main) in a linear history —
the branch IS the commit's ancestor, so there are zero divergent commits.

Fix: HEAD="${{ github.sha }}" — both BASE and HEAD are SHAs, so the API
returns the pushed commits' changed files.

Verified: compare/{before_sha}...{current_sha} correctly returns pushed commits
on a linear push. For PR events, branch names still work correctly (the Compare
API handles branch-vs-branch comparisons fine).

Closes regression introduced in PR #497.

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

.gitea/workflows/harness-replays.yml

@@ -87,20 +87,19 @@ jobs:
           # Determine base and head refs for the Compare API call.
           # Gitea Compare API accepts branch names OR commit SHAs as base/head.
           # Pull request: base.ref + head.ref are in the event payload (branch names).
-          # Push: github.event.before (SHA of previous tip) as BASE, $GITHUB_REF
-          #       (branch name) as HEAD. These are different, so the Compare API
-          #       returns the actual diff — unlike the broken form which set both
-          #       BASE and HEAD to the same branch name, making
-          #       "compare/main...main" always return zero files.
+          # Push: github.event.before (SHA of previous tip) as BASE, github.sha
+          #       (current tip SHA) as HEAD. Both SHAs means the API returns the
+          #       pushed commits (unlike branch-name HEAD which returns empty in
+          #       linear-history push events — Gitea treats SHA-vs-branch as
+          #       "show divergent commits" and a linear push has zero of those).
           if [ "${{ github.event_name }}" = "pull_request" ]; then
             BASE="${{ github.event.pull_request.base.ref }}"
             HEAD="${{ github.event.pull_request.head.ref }}"
           elif [ -n "${{ github.event.before }}" ] && \
                ! echo "${{ github.event.before }}" | grep -qE '^0+$'; then
-            # Push event: BASE = previous tip (SHA), HEAD = current branch name.
+            # Push event: BASE = previous tip (SHA), HEAD = current tip (SHA).
             BASE="${{ github.event.before }}"
-            HEAD_REF="${GITHUB_REF#refs/heads/}"
-            HEAD="${HEAD_REF:-main}"
+            HEAD="${{ github.sha }}"
           else
             # New branch or github.event.before unavailable — run everything.
             echo "run=true" >> "$GITHUB_OUTPUT"