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/agent/loop.py

@@ -34,7 +34,9 @@ from nanobot.config.schema import AgentDefaults, ModelPresetConfig
 from nanobot.providers.base import LLMProvider
 from nanobot.providers.factory import ProviderSnapshot
 from nanobot.session.goal_state import (
+    goal_state_runtime_lines,
     runner_wall_llm_timeout_s,
+    sustained_goal_active,
 )
 from nanobot.session.manager import Session, SessionManager
 from nanobot.session.webui_turns import (
@@ -47,7 +49,10 @@ from nanobot.utils.helpers import image_placeholder_text
 from nanobot.utils.helpers import truncate_text as truncate_text_fn
 from nanobot.utils.image_generation_intent import image_generation_prompt
 from nanobot.utils.llm_runtime import LLMRuntime
-from nanobot.utils.runtime import EMPTY_FINAL_RESPONSE_MESSAGE
+from nanobot.utils.runtime import (
+    EMPTY_FINAL_RESPONSE_MESSAGE,
+    SUSTAINED_GOAL_CONTINUE_PROMPT,
+)
 
 if TYPE_CHECKING:
     from nanobot.config.schema import (
@@ -729,6 +734,15 @@ class AgentLoop:
 
         active_session_key = session.key if session else session_key
         file_state_token = bind_file_states(self._file_state_store.for_session(active_session_key))
+        # Build continuation message that embeds the active goal objective so
+        # the LLM can see it even if earlier Runtime Context was truncated.
+        _goal_lines = goal_state_runtime_lines(session.metadata if session is not None else None)
+        _goal_continue = (
+            "You have an active sustained goal:\n\n"
+            + "\n".join(_goal_lines)
+            + "\n\nPlease continue working toward the objective using your tools, "
+            "or call complete_goal if the work is truly finished."
+        ) if _goal_lines else SUSTAINED_GOAL_CONTINUE_PROMPT
         try:
             result = await self.runner.run(AgentRunSpec(
                 initial_messages=initial_messages,
@@ -756,6 +770,8 @@ class AgentLoop:
                     session.key if session is not None else session_key,
                     metadata=(session.metadata if session is not None else None),
                 ),
+                goal_active_predicate=lambda: sustained_goal_active(session.metadata) if session is not None else False,
+                goal_continue_message=_goal_continue,
             ))
         finally:
             reset_file_states(file_state_token)

nanobot/agent/runner.py

@@ -8,7 +8,7 @@ import os
 from contextlib import suppress
 from dataclasses import dataclass, field
 from pathlib import Path
-from typing import Any
+from typing import Any, Callable
 
 from loguru import logger
 
@@ -42,6 +42,7 @@ from nanobot.utils.prompt_templates import render_template
 from nanobot.utils.runtime import (
     EMPTY_FINAL_RESPONSE_MESSAGE,
     build_finalization_retry_message,
+    build_goal_continue_message,
     build_length_recovery_message,
     ensure_nonempty_tool_result,
     is_blank_text,
@@ -97,6 +98,8 @@ class AgentRunSpec:
     checkpoint_callback: Any | None = None
     injection_callback: Any | None = None
     llm_timeout_s: float | None = None
+    goal_active_predicate: Callable[[], bool] | None = None
+    goal_continue_message: str | None = None
 
 
 @dataclass(slots=True)
@@ -167,6 +170,7 @@ class AgentRunner:
         *,
         phase: str = "after error",
         iteration: int | None = None,
+        allow_goal_continue: bool = False,
     ) -> tuple[bool, int]:
         """Drain pending injections. Returns (should_continue, updated_cycles).
 
@@ -178,6 +182,10 @@ class AgentRunner:
         if injection_cycles >= _MAX_INJECTION_CYCLES:
             return False, injection_cycles
         injections = await self._drain_injections(spec)
+        if not injections and allow_goal_continue and assistant_message is not None:
+            predicate = spec.goal_active_predicate
+            if predicate is not None and predicate():
+                injections = [build_goal_continue_message(spec.goal_continue_message)]
         if not injections:
             return False, injection_cycles
         injection_cycles += 1
@@ -475,6 +483,7 @@ class AgentRunner:
                 spec, messages, assistant_message, injection_cycles,
                 phase="after final response",
                 iteration=iteration,
+                allow_goal_continue=True,
             )
             if should_continue:
                 had_injections = True