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.go

@@ -32,3 +32,82 @@ func (c *Client) GetFileContents(ctx context.Context, owner, repo, path, ref str
 	}
 	return &fc, nil
 }
+
+type Branch struct {
+	Name   string `json:"name"`
+	Commit struct {
+		ID  string `json:"id"`
+		URL string `json:"url"`
+	} `json:"commit"`
+}
+
+// BranchExists returns (true, nil) if the branch exists, (false, nil) on 404, (false, err) otherwise.
+func (c *Client) BranchExists(ctx context.Context, owner, repo, branch string) (bool, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/branches/%s", owner, repo, branch)
+	body, status, err := c.GetJSON(ctx, p)
+	if err != nil {
+		return false, err
+	}
+	if status == 404 {
+		return false, nil
+	}
+	if err := MapStatus(status, body); err != nil {
+		return false, err
+	}
+	return true, nil
+}
+
+func (c *Client) CreateBranch(ctx context.Context, owner, repo, newBranch, oldBranch string) error {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/branches", owner, repo)
+	payload, err := json.Marshal(map[string]string{
+		"new_branch_name": newBranch,
+		"old_branch_name": oldBranch,
+	})
+	if err != nil {
+		return err
+	}
+	body, status, err := c.PostJSON(ctx, p, payload)
+	if err != nil {
+		return err
+	}
+	return MapStatus(status, body)
+}
+
+type UpsertFileArgs struct {
+	Branch  string `json:"branch"`
+	Content string `json:"content"` // already base64-encoded
+	Message string `json:"message"`
+	Sha     string `json:"sha,omitempty"`
+}
+
+type FileWriteResult struct {
+	Content struct {
+		Path    string `json:"path"`
+		Sha     string `json:"sha"`
+		HTMLURL string `json:"html_url"`
+	} `json:"content"`
+	Commit struct {
+		Sha     string `json:"sha"`
+		HTMLURL string `json:"html_url"`
+	} `json:"commit"`
+}
+
+func (c *Client) UpsertFile(ctx context.Context, owner, repo, path string, args UpsertFileArgs) (*FileWriteResult, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", owner, repo, path)
+	payload, err := json.Marshal(args)
+	if err != nil {
+		return nil, err
+	}
+	body, status, err := c.PutJSON(ctx, p, payload)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var out FileWriteResult
+	if err := json.Unmarshal(body, &out); err != nil {
+		return nil, err
+	}
+	return &out, nil
+}