fix(merge gate): an unknown head commit never merges; pr_merge refuses closed and draft PRs (review of PR #76)

Review found a bypass: a PR reporting an empty head sha made the runs query lose its head_sha filter (the repo's 50
most recent runs of any commit were classified) and dropped head_commit_id (omitempty), so the merge was unbound.
Both tools now go through evaluateMergeGate, which refuses without a head sha, and pollRuns makes no query without
one. pr_merge also reports merged:false for a PR that is not open or is a draft (Gitea refuses closed ones itself;
the gate no longer depends on that). Tests: unit (gate, counting-server pollRuns with positive control), end to end
for both tools, CI scoped to head_sha, closed/merged/draft. The two caller-wiring mutants are equivalent to the
pollRuns guard and survive by design.

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

internal/tools/tbd_ship_internal_test.go

@@ -1,6 +1,11 @@
 package tools
 
 import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"sync/atomic"
 	"testing"
 
 	"git.d-ma.be/mathias/gitea-mcp/internal/gitea"
@@ -98,3 +103,36 @@ func TestClassifyRisk(t *testing.T) {
 		})
 	}
 }
+
+// The merge gate needs a head commit to scope CI to and to bind the merge to: without one nothing merges.
+func TestEvaluateMergeGate_UnknownHeadCommitNeverMerges(t *testing.T) {
+	green := []gitea.WorkflowRun{{Status: "completed", Conclusion: "success"}}
+	g := evaluateMergeGate("", green, &gitea.BranchProtection{}, riskLow)
+	if g.merge {
+		t.Fatalf("merge = true for an empty head sha (reason %q)", g.reason)
+	}
+	if !strings.Contains(g.reason, "head commit") {
+		t.Errorf("reason %q should name the head commit", g.reason)
+	}
+	if ok := evaluateMergeGate("abc", green, &gitea.BranchProtection{}, riskLow); !ok.merge {
+		t.Errorf("a known head sha with green CI and risk/low must still merge, got %q", ok.reason)
+	}
+}
+
+func TestPollRuns_NoShaMakesNoQuery(t *testing.T) {
+	var requests atomic.Int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requests.Add(1)
+		_, _ = w.Write([]byte(`{"total_count":1,"workflow_runs":[{"id":1,"status":"completed","conclusion":"success"}]}`))
+	}))
+	defer srv.Close()
+	c := gitea.NewClient(srv.URL, "tok")
+
+	if runs := pollRuns(context.Background(), c, "o", "r", "", 0); len(runs) != 0 || requests.Load() != 0 {
+		t.Errorf("an empty sha must make no request and return no runs, got %d runs after %d requests", len(runs), requests.Load())
+	}
+	// positive control: with a sha the same server is asked, so the zero above means something
+	if runs := pollRuns(context.Background(), c, "o", "r", "abc", 0); len(runs) != 1 || requests.Load() != 1 {
+		t.Errorf("control: with a sha expected 1 run after 1 request, got %d runs after %d requests", len(runs), requests.Load())
+	}
+}