fix(ci): assert the focused path's shard shape in the workflow contract

The workflow contract pinned the focused path by literal: the job name
`Test (changed packages)`, the unsharded
`list "$TEST_BASE_REF" "$TEST_HEAD_REF"` call, and a single
`go test -timeout=15m` line standing in for internal/app's package-level
headroom. Sharding the job changed all three literals, so `Test (workflow
and release contracts)` failed on this branch even though every shard
selection test passed.

Each invariant the contract guarded still holds, so the assertions are
updated to the new shape rather than relaxed:

- the focused job must still exist, now as the matrix job, named the way
  the contract already names `Test (race: ${{ matrix.shard }})`;
- package selection must still derive from the authoritative synthetic
  merge base/head, now with an explicit shard argument, so pointing it at
  any other ref still fails the contract;
- internal/app's headroom is asserted through the process-isolating
  helper and the per-shard budgets, mirroring the assertions already
  applied to test-race. That is stronger than the old single -timeout: it
  pins the mechanism that keeps the suite inside its budget rather than
  the number alone. release-scripts membership is asserted too, because
  its dedicated job only runs at full-suite or release-sensitive scope,
  so losing it here would silently stop testing test/scripts changes.

The shard comparisons in the focused job are quoted so that job reads
verbatim like test-race's.

Ablating the implementation one change at a time turns the contract red
in all five cases: removing the app helper call, dropping release-scripts
from the matrix, selecting from HEAD~1, collapsing the matrix back to a
single unsharded job, and dropping the cli/smoke timeout budget.

test/scripts/changelog_pr_gate_test.go

@@ -1151,13 +1151,13 @@ func TestChangelogPRFastPathWorkflowContract(t *testing.T) {
 		"filename.startsWith('pkg/cmdutil/')",
 		"filename === 'skills_embed.go'",
 		"filename.startsWith('test/mock_mcp/')",
-		"name: Test (changed packages)",
+		`name: "Test (focused: ${{ matrix.shard }})"`,
 		"changed-test-packages.sh",
 		"Verify authoritative synthetic merge",
 		`test "$(git rev-parse HEAD^1)" = "$PR_BASE_SHA"`,
 		`test "$(git rev-parse HEAD^2)" = "$PR_HEAD_SHA"`,
 		`echo "TEST_HEAD_REF=$(git rev-parse HEAD)"`,
-		"list \"$TEST_BASE_REF\" \"$TEST_HEAD_REF\"",
+		`list-shard "$TEST_SHARD" "$TEST_BASE_REF" "$TEST_HEAD_REF"`,
 		"needs.lint.outputs.full_suite != 'true'",
 		`name: "Test (race: ${{ matrix.shard }})"`,
 		"name: Test (workflow and release contracts)",
@@ -1217,8 +1217,30 @@ func TestChangelogPRFastPathWorkflowContract(t *testing.T) {
 	if !strings.Contains(focusedJob, "timeout-minutes: 20") {
 		t.Error("focused test job must allow the scoped race suite up to 20 minutes")
 	}
-	if !strings.Contains(focusedJob, `go test -v -race -count=1 -timeout=15m "${packages[@]}"`) {
-		t.Error("focused race tests must retain enough package-level time for internal/app")
+	// The focused path fans the impacted set across the same shards as test-race
+	// and runs each shard the way test-race runs it, so no single job carries
+	// internal/app together with its reverse dependencies. internal/app keeps its
+	// package-level headroom through the process-isolating helper instead of one
+	// long -timeout, which is strictly stronger: every process releases the
+	// framework registries it populated. release-scripts is asserted because its
+	// dedicated job only runs at full-suite or release-sensitive scope, so losing
+	// it here would silently stop testing test/scripts changes.
+	for _, want := range []string{
+		`if [ "$TEST_SHARD" = "app" ]; then`,
+		`test "${#packages[@]}" -eq 1`,
+		`./scripts/ci/run-app-race-tests.sh run "${packages[0]}"`,
+		`if [ "$TEST_SHARD" = "release-scripts" ]; then`,
+		`go test -v -count=1 -timeout=10m "${packages[@]}"`,
+		"timeout_budget=12m",
+		`if [ "$TEST_SHARD" = "cli" ] ||`,
+		`[ "$TEST_SHARD" = "smoke" ]; then`,
+		"timeout_budget=15m",
+		`go test -v -race -count=1 -timeout="$timeout_budget" "${packages[@]}"`,
+		"- release-scripts",
+	} {
+		if !strings.Contains(focusedJob, want) {
+			t.Errorf("focused test job missing shard contract %q", want)
+		}
 	}
 
 	raceStart := focusedEnd