fix(git-tools): pr-merge queue guard reads CI status from the BASE repo (fork PRs, B1)

For a fork PR, pr-metadata's headRepository names the fork (e.g.
stack-mos-dt-0/stack), and pr-merge passed it to ci-queue-wait via -R.
Woodpecker posts commit statuses on the BASE repository, so the guard read
statuses:null from the fork and every fork merge aborted with
state=malformed rc=3 (#1215, gate-merge-01 B1; standalone queue-wait
against the base repo returns terminal-success on the same sha). Same
family as d339e8fd.

- pr-metadata.sh: normalize baseRepository from Gitea base.repo.full_name
- pr-merge.sh: pass the BASE repo to the queue guard; head repo remains
  for head-sha identity; origin-repo fallback when metadata carries no
  base repository (correct for same-repo PRs)
- test-pr-merge-fork-ci-status.sh: fork-PR regression, CI-reachable via
  test:framework-shell beside its siblings; asserts the guard NEVER sees
  the fork and gets the base repo, head branch and exact sha
- test-pr-merge-queue-branch.sh: its old -R assertion pinned the fork
  repo, i.e. the bug; now asserts the base repo

packages/mosaic/framework/tools/git/test-pr-merge-queue-branch.sh

@@ -15,7 +15,7 @@ cp "$SCRIPT_DIR/detect-platform.sh" "$FIXTURE_DIR/detect-platform.sh"
 
 cat > "$FIXTURE_DIR/pr-metadata.sh" <<'SH'
 #!/usr/bin/env bash
-printf '%s\n' '{"baseRefName":"main","headRefName":"fix/rm-03-fixture","headRefOid":"0123456789abcdef0123456789abcdef01234567","headRepository":"contributor/widgets-fork"}'
+printf '%s\n' '{"baseRefName":"main","baseRepository":"mosaicstack/stack","headRefName":"fix/rm-03-fixture","headRefOid":"0123456789abcdef0123456789abcdef01234567","headRepository":"contributor/widgets-fork"}'
 SH
 
 cat > "$FIXTURE_DIR/ci-queue-wait.sh" <<'SH'
@@ -52,8 +52,13 @@ if grep -q -- '-B main' "$CALL_LOG"; then
     cat "$CALL_LOG" >&2
     exit 1
 fi
-if ! grep -q -- '-R contributor/widgets-fork' "$CALL_LOG"; then
-    echo "FAIL: merge queue guard did not receive the fork head repository" >&2
+if ! grep -q -- '-R mosaicstack/stack' "$CALL_LOG"; then
+    echo "FAIL: merge queue guard did not receive the BASE repository for CI status" >&2
+    cat "$CALL_LOG" >&2
+    exit 1
+fi
+if grep -q -- '-R contributor/widgets-fork' "$CALL_LOG"; then
+    echo "FAIL: merge queue guard received the fork head repository (B1: statuses are posted on the base repo)" >&2
     cat "$CALL_LOG" >&2
     exit 1
 fi