feat: add status watcher to MCP server (#18320)

This is meant to complement the existing task reporter since the LLM
does not call it reliably.

It also includes refactoring to use the common agent flags/env vars.

cli/gitaskpass.go

@@ -33,7 +33,7 @@ func (r *RootCmd) gitAskpass() *serpent.Command {
 				return xerrors.Errorf("parse host: %w", err)
 			}
 
-			client, err := r.createAgentClient()
+			client, err := r.tryCreateAgentClient()
 			if err != nil {
 				return xerrors.Errorf("create agent client: %w", err)
 			}