feat(tools): file_write_branch

Add BranchExists/CreateBranch/UpsertFile gitea client methods and the
file_write_branch MCP tool. Branch is auto-created from base (or repo
default_branch) when it doesn't exist; file is upserted via PUT contents.

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

internal/gitea/files_test.go

@@ -2,6 +2,8 @@ package gitea_test
 
 import (
 	"context"
+	"encoding/json"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"testing"
@@ -28,3 +30,89 @@ func TestGetFileContents(t *testing.T) {
 	assert.Equal(t, int64(13), fc.Size)
 	assert.Equal(t, "SGVsbG8sIHdvcmxkIQ==", fc.Content)
 }
+
+func TestBranchExistsTrue(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/branches/main", r.URL.Path)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"name":"main","commit":{"id":"abc123","url":"http://example.com"}}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	exists, err := c.BranchExists(context.Background(), "o", "r", "main")
+	require.NoError(t, err)
+	assert.True(t, exists)
+}
+
+func TestBranchExistsFalseOn404(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/branches/nonexistent", r.URL.Path)
+		w.WriteHeader(http.StatusNotFound)
+		_, _ = w.Write([]byte(`{"message":"branch not found"}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	exists, err := c.BranchExists(context.Background(), "o", "r", "nonexistent")
+	require.NoError(t, err)
+	assert.False(t, exists)
+}
+
+func TestCreateBranchSendsPayload(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/branches", r.URL.Path)
+		assert.Equal(t, http.MethodPost, r.Method)
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.WriteHeader(http.StatusCreated)
+		_, _ = w.Write([]byte(`{"name":"feat/x","commit":{"id":"abc","url":"http://example.com"}}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	err := c.CreateBranch(context.Background(), "o", "r", "feat/x", "main")
+	require.NoError(t, err)
+
+	var payload map[string]string
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "feat/x", payload["new_branch_name"])
+	assert.Equal(t, "main", payload["old_branch_name"])
+}
+
+func TestUpsertFileSendsPayloadAndDecodesResult(t *testing.T) {
+	var captured []byte
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		assert.Equal(t, "/api/v1/repos/o/r/contents/p.md", r.URL.Path)
+		assert.Equal(t, http.MethodPut, r.Method)
+		var err error
+		captured, err = io.ReadAll(r.Body)
+		require.NoError(t, err)
+		w.WriteHeader(http.StatusCreated)
+		_, _ = w.Write([]byte(`{"content":{"path":"p.md","sha":"newsha","html_url":"http://example.com/p.md"},"commit":{"sha":"abc","html_url":"http://example.com/commit/abc"}}`))
+	}))
+	defer srv.Close()
+
+	c := gitea.NewClient(srv.URL, "tok")
+	result, err := c.UpsertFile(context.Background(), "o", "r", "p.md", gitea.UpsertFileArgs{
+		Branch:  "feat/x",
+		Content: "aGVsbG8=",
+		Message: "add p.md",
+		Sha:     "oldsha",
+	})
+	require.NoError(t, err)
+
+	var payload map[string]string
+	require.NoError(t, json.Unmarshal(captured, &payload))
+	assert.Equal(t, "feat/x", payload["branch"])
+	assert.Equal(t, "aGVsbG8=", payload["content"])
+	assert.Equal(t, "add p.md", payload["message"])
+	assert.Equal(t, "oldsha", payload["sha"])
+
+	assert.Equal(t, "p.md", result.Content.Path)
+	assert.Equal(t, "newsha", result.Content.Sha)
+	assert.Equal(t, "http://example.com/p.md", result.Content.HTMLURL)
+	assert.Equal(t, "abc", result.Commit.Sha)
+}