feat(tools): file_read with default-branch resolution

Adds GetFileContents to the gitea client and a file_read MCP tool.
When ref is omitted, the tool resolves the repo default_branch via
GetRepo before fetching contents. Decoded content capped at 1 MiB.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gitea/files_test.go

@@ -0,0 +1,30 @@
+package gitea_test
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"gitea.d-ma.be/mathias/gitea-mcp/internal/gitea"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestGetFileContents(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/mathias/infra/contents/README.md", r.URL.Path)
+		assert.Equal(t, "main", r.URL.Query().Get("ref"))
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"path":"README.md","sha":"deadbeef","size":13,"content":"SGVsbG8sIHdvcmxkIQ==","encoding":"base64"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	fc, err := c.GetFileContents(context.Background(), "mathias", "infra", "README.md", "main")
+	require.NoError(t, err)
+	assert.Equal(t, "README.md", fc.Path)
+	assert.Equal(t, "deadbeef", fc.Sha)
+	assert.Equal(t, int64(13), fc.Size)
+	assert.Equal(t, "SGVsbG8sIHdvcmxkIQ==", fc.Content)
+}