fix(pr_merge): advertise canonical number, demote index to alias (#45)

pr_merge was the only tool still advertising `index` as its id field while
every other issue/PR tool uses the canonical `number` (#38). Flipped its
schema property + required + struct field/tag `index` -> `number`; the existing
`index`->`number` shim keeps legacy `index` callers working, so no shim change
was needed (no canonical-`index` tool remains). Now the id arg is `number`
uniformly across all per-issue/PR tools.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/tools/pr_merge_test.go

@@ -63,6 +63,30 @@ func TestPRMergeConflictReturnsError(t *testing.T) {
 	assert.ErrorIs(t, err, gitea.ErrConflict)
 }
 
+// #45: pr_merge advertises the canonical `number` (was `index`); `index` stays
+// an accepted alias via the shim.
+func TestPRMergeNumberCanonical(t *testing.T) {
+	sch := string(tools.NewPRMerge(gitea.NewClient("http://unused", ""), allowlist.New([]string{"owner"})).Descriptor().InputSchema)
+	assert.Contains(t, sch, `"number":`, "pr_merge must advertise number")
+	assert.NotContains(t, sch, `"index":`, "pr_merge must not advertise index")
+
+	for _, args := range []string{
+		`{"owner":"owner","repo":"repo","number":7}`,
+		`{"owner":"owner","repo":"repo","index":7}`,
+	} {
+		var gotPath string
+		srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			gotPath = r.URL.Path
+			w.WriteHeader(http.StatusNoContent)
+		}))
+		tool := tools.NewPRMerge(gitea.NewClient(srv.URL, "tok"), allowlist.New([]string{"owner"}))
+		_, err := tool.Call(context.Background(), json.RawMessage(args))
+		require.NoError(t, err, args)
+		assert.Equal(t, "/api/v1/repos/owner/repo/pulls/7/merge", gotPath, args)
+		srv.Close()
+	}
+}
+
 func TestPRMergeAllowlistRejects(t *testing.T) {
 	tool := tools.NewPRMerge(gitea.NewClient("http://unused", ""), allowlist.New([]string{"allowed"}))
 	_, err := tool.Call(context.Background(), json.RawMessage(`{"owner":"evil","name":"repo","index":1}`))