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/client.go

@@ -6,22 +6,40 @@ import (
 	"io"
 	"net/http"
 	"time"
+
+	"github.com/hashicorp/golang-lru/v2/expirable"
 )
 
 type Client struct {
-	baseURL string
-	token   string
-	hc      *http.Client
+	baseURL     string
+	token       string
+	hc          *http.Client
+	branchCache *expirable.LRU[string, string]
 }
 
 func NewClient(baseURL, token string) *Client {
 	return &Client{
-		baseURL: baseURL,
-		token:   token,
-		hc:      &http.Client{Timeout: 30 * time.Second},
+		baseURL:     baseURL,
+		token:       token,
+		hc:          &http.Client{Timeout: 30 * time.Second},
+		branchCache: expirable.NewLRU[string, string](64, nil, 60*time.Second),
 	}
 }
 
+// DefaultBranch returns the default branch for a repo. Cached for 60s.
+func (c *Client) DefaultBranch(ctx context.Context, owner, name string) (string, error) {
+	key := owner + "/" + name
+	if v, ok := c.branchCache.Get(key); ok {
+		return v, nil
+	}
+	repo, err := c.GetRepo(ctx, owner, name)
+	if err != nil {
+		return "", err
+	}
+	c.branchCache.Add(key, repo.DefaultBranch)
+	return repo.DefaultBranch, nil
+}
+
 func (c *Client) doOnce(ctx context.Context, method, path string, body []byte) ([]byte, int, error) {
 	var reader io.Reader
 	if body != nil {

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")
+}