fix(agent): prevent runner from exiting while sustained goal is active

`long_task` registers a sustained objective, but `AgentRunner` would
still exit with `stop_reason="completed"` when the LLM produced a final
text response without calling `complete_goal`. This defeated the purpose
of sustained goals.

Add `goal_active_predicate` and `goal_continue_message` to `AgentRunSpec`.
When the predicate returns `True` at the natural completion checkpoint,
inject a continuation message via the existing `_try_drain_injections`
machinery, forcing the runner to continue looping.

Also extract the default continuation prompt to
`nanobot/utils/runtime.py` alongside the existing recovery-message
builders.

nanobot/utils/runtime.py

@@ -29,6 +29,11 @@ LENGTH_RECOVERY_PROMPT = (
     "— no recap, no apology. Break remaining work into smaller steps if needed."
 )
 
+SUSTAINED_GOAL_CONTINUE_PROMPT = (
+    "You have an active sustained goal. Please continue working toward the "
+    "objective using your tools, or call complete_goal if the work is truly finished."
+)
+
 
 def empty_tool_result_message(tool_name: str) -> str:
     """Short prompt-safe marker for tools that completed without visible output."""
@@ -65,6 +70,11 @@ def build_length_recovery_message() -> dict[str, str]:
     return {"role": "user", "content": LENGTH_RECOVERY_PROMPT}
 
 
+def build_goal_continue_message(custom: str | None = None) -> dict[str, str]:
+    """Prompt the model to continue when a sustained goal is still active."""
+    return {"role": "user", "content": custom or SUSTAINED_GOAL_CONTINUE_PROMPT}
+
+
 def external_lookup_signature(tool_name: str, arguments: dict[str, Any]) -> str | None:
     """Stable signature for repeated external lookups we want to throttle."""
     if tool_name == "web_fetch":