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>

cmd/gitea-mcp/main.go

@@ -30,6 +30,7 @@ func main() {
 	reg.Register(tools.NewRepoList(giteaClient, ownerAllow))
 	reg.Register(tools.NewRepoGet(giteaClient, ownerAllow))
 	reg.Register(tools.NewFileRead(giteaClient, ownerAllow))
+	reg.Register(tools.NewFileWriteBranch(giteaClient, ownerAllow))
 
 	mcpSrv := mcp.NewServer(mcp.ServerOptions{
 		Registry: reg,