require a2a pack write prefix support

agent.py

@@ -81,6 +81,7 @@ class AgentBuilder(A2AAgent[BuilderConfig, NoAuth]):
         grant_mode="read_write_overlay",
         grant_allow_patterns=("agents/{name}/**",),
         grant_outputs_prefix="agents/{name}/",
+        grant_write_prefixes=("agents/{name}/",),
         grant_ttl_seconds=1800,
         grant_run_timeout_seconds=1740,
         grant_approval_timeout_seconds=180,
@@ -277,4 +278,4 @@ def _find_url(value: Any) -> str | None:
         m = _URL_RE.search(value)
         return m.group(0) if m else None
     return None
-# rebuild against a2a-pack 0.1.17 for nullable LLM extra_body handling
+# rebuild against a2a-pack 0.1.29 for negotiated write prefixes