fix(merge gate): bind pr_merge and tbd_ship merges to the head commit the gate checked (TOCTOU)

The gate evaluates CI for one head sha; a push to the PR branch between that check and the merge would have merged
unchecked code. MergePRArgs now carries head_commit_id (documented in Gitea 1.27.1's MergePullRequestOption) and both
tools send the sha they evaluated. Tests for both tools; each removal flip-tested red.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SZkhi49hfeSqBp5aZJNXsM

internal/tools/pr_merge.go

@@ -97,9 +97,10 @@ func (t *PRMerge) Call(ctx context.Context, raw json.RawMessage) (json.RawMessag
 	}
 
 	if err := t.c.MergePullRequest(ctx, args.Owner, args.Repo, args.Number, gitea.MergePRArgs{
-		Do:    style,
-		Title: args.Title,
-		Body:  args.Body,
+		Do:           style,
+		Title:        args.Title,
+		Body:         args.Body,
+		HeadCommitID: pr.Head.Sha,
 	}); err != nil {
 		return nil, err
 	}

internal/tools/pr_merge_test.go

@@ -145,6 +145,19 @@ func TestPRMergeDefaultsToMergeStyleAndPassesStyleThrough(t *testing.T) {
 	assert.Equal(t, "squash", payload["Do"])
 }
 
+// TOCTOU: the gate read CI for ONE head commit. The merge must be bound to that commit, so a push to the PR branch
+// after the check makes Gitea refuse the merge instead of merging code nobody checked.
+func TestPRMerge_TheMergeIsBoundToTheCommitTheGateChecked(t *testing.T) {
+	f := newPRMergeFake(t, prMergeFakeOpts{})
+
+	_, err := callMerge(t, f, mergeArgs)
+
+	require.NoError(t, err)
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal(f.mergeBody, &payload))
+	assert.Equal(t, "abc", payload["head_commit_id"], "the head sha the CI check used")
+}
+
 // A merge conflict is still an error, as before the gate existed.
 func TestPRMergeConflictReturnsError(t *testing.T) {
 	f := newPRMergeFake(t, prMergeFakeOpts{mergeStatus: http.StatusConflict})

internal/tools/tbd_ship.go

@@ -320,7 +320,7 @@ func (t *TBDShip) Call(ctx context.Context, raw json.RawMessage) (json.RawMessag
 	}
 
 	// Green + unprotected → squash-merge, then delete the short-lived branch.
-	if err := t.c.MergePullRequest(ctx, args.Owner, args.Repo, pr.Number, gitea.MergePRArgs{Do: "squash"}); err != nil {
+	if err := t.c.MergePullRequest(ctx, args.Owner, args.Repo, pr.Number, gitea.MergePRArgs{Do: "squash", HeadCommitID: pr.Head.Sha}); err != nil {
 		if errors.Is(err, gitea.ErrConflict) {
 			result["reason"] = "base moved and the merge is not clean — PR left open, not merged; resolve the conflict and merge manually"
 			return textOK(result)

internal/tools/tbd_ship_test.go

@@ -3,6 +3,7 @@ package tools_test
 import (
 	"context"
 	"encoding/json"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -63,6 +64,8 @@ func shipFakeFull(t *testing.T, runsJSON string, requiredApprovals, mergeStatus
 		case r.Method == http.MethodGet && strings.Contains(p, "/actions/runs"):
 			_, _ = w.Write([]byte(`{"total_count":0,"workflow_runs":` + runsJSON + `}`))
 		case r.Method == http.MethodPost && strings.HasSuffix(p, "/pulls/7/merge"):
+			body, _ := io.ReadAll(r.Body)
+			shipLastMergeBody.Store(string(body))
 			merged.Store(true)
 			w.WriteHeader(mergeStatus)
 			_, _ = w.Write([]byte(`{}`))
@@ -216,6 +219,23 @@ func TestTBDShip_Resume_ExistingBranchAndPR(t *testing.T) {
 	assert.True(t, merged.Load())
 }
 
+// shipLastMergeBody is the body of the most recent merge request the fake received.
+var shipLastMergeBody atomic.Value
+
+// TOCTOU: the merge is bound to the head commit whose CI the gate evaluated (gitea-mcp#74).
+func TestTBDShip_TheMergeIsBoundToTheCommitTheGateChecked(t *testing.T) {
+	var merged, deleted atomic.Bool
+	srv := shipFake(t, shipGreen, &merged, &deleted)
+	defer srv.Close()
+
+	res := callShip(t, srv.URL, shipArgs)
+
+	require.Equal(t, true, res["merged"])
+	var payload map[string]any
+	require.NoError(t, json.Unmarshal([]byte(shipLastMergeBody.Load().(string)), &payload))
+	assert.Equal(t, "abc", payload["head_commit_id"], "the head sha the CI check used")
+}
+
 // ---- gitea-mcp#63: the risk tier is an input to the gate. Only exactly one risk/low label merges. ----
 const (
 	shipGreen = `[{"id":1,"status":"completed","conclusion":"success","head_sha":"abc"}]`