feat(gitea): default-branch lru cache

Shared LRU avoids repeated Gitea calls for default-branch resolution;
the simple stdlib map alternative would race on concurrent access without
a mutex per entry, which is more code than the LRU.

internal/gitea/repos_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"net/http"
 	"net/http/httptest"
+	"sync/atomic"
 	"testing"
 
 	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
@@ -45,3 +46,23 @@ func TestListRepos(t *testing.T) {
 	assert.Equal(t, "mathias/infra", repos[0].FullName)
 	assert.Equal(t, "main", repos[0].DefaultBranch)
 }
+
+func TestDefaultBranchCachesAcrossCalls(t *testing.T) {
+	var hits int32
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		atomic.AddInt32(&hits, 1)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"name":"infra","full_name":"o/infra","default_branch":"trunk"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+
+	for i := 0; i < 5; i++ {
+		b, err := c.DefaultBranch(context.Background(), "o", "infra")
+		require.NoError(t, err)
+		assert.Equal(t, "trunk", b)
+	}
+
+	assert.Equal(t, int32(1), atomic.LoadInt32(&hits), "5 calls should cause exactly 1 server hit due to cache")
+}