fix(cron): state per-action requirements in descriptions, keep list/remove callable

The previous patch promoted `message` into top-level `required`, which solved
the `add` loop but broke `list` and `remove`: `ToolRegistry.prepare_call`
enforces `required` via `validate_params`, so `cron(action="list")` and
`cron(action="remove", job_id=...)` — both documented in `SKILL.md` — started
failing schema validation with the same "missing required message" shape that
#3113 describes for `add`.

Instead:
- Keep `required=["action"]` so `list`/`remove` stay callable.
- Prefix `message`'s description with `REQUIRED when action='add'.` and
  `job_id`'s with `REQUIRED when action='remove'.` so LLMs see the real
  per-action contract up front.
- Keep the improved runtime error message from the previous commit for the
  case an LLM still omits `message` on `add`.

Also add `tests/cron/test_cron_tool_schema_contract.py` to lock in:
  - `list` and `remove` pass schema validation with no `message`
  - `add` with `message` passes
  - `add` without `message` surfaces the actionable runtime error
  - field descriptions carry the REQUIRED hints
  - top-level `required` stays `["action"]`

Existing `tests/cron/test_cron_tool_list.py` cases bypass schema validation by
calling `_list_jobs()` / `_remove_job()` directly, which is why CI didn't catch
the regression; the new test goes through `ToolRegistry.prepare_call`.

nanobot/agent/tools/cron.py

@@ -18,8 +18,9 @@ from nanobot.cron.types import CronJob, CronJobState, CronSchedule
             "(e.g., 'weather-monitor', 'daily-standup'). Defaults to first 30 chars of message."
         ),
         message=StringSchema(
-            "Instruction for the agent to execute when the job triggers "
-            "(e.g., 'Send a reminder to WeChat: xxx' or 'Check system status and report')"
+            "REQUIRED when action='add'. Instruction for the agent to execute when the job triggers "
+            "(e.g., 'Send a reminder to WeChat: xxx' or 'Check system status and report'). "
+            "Not used for action='list' or action='remove'."
         ),
         every_seconds=IntegerSchema(0, description="Interval in seconds (for recurring tasks)"),
         cron_expr=StringSchema("Cron expression like '0 9 * * *' (for scheduled tasks)"),
@@ -35,8 +36,8 @@ from nanobot.cron.types import CronJob, CronJobState, CronSchedule
             description="Whether to deliver the execution result to the user channel (default true)",
             default=True,
         ),
-        job_id=StringSchema("Job ID (for remove)"),
-        required=["action", "message"],
+        job_id=StringSchema("REQUIRED when action='remove'. Job ID to remove (obtain via action='list')."),
+        required=["action"],
     )
 )
 class CronTool(Tool):

tests/cron/test_cron_tool_schema_contract.py

@@ -0,0 +1,99 @@
+"""Regression tests for the cron tool's JSON-schema / runtime contract (#3113).
+
+The schema advertised ``required=["action"]`` while ``_add_job`` rejected empty
+``message``; LLMs rationally omitted ``message`` and looped on the runtime
+error. The fix keeps ``required=["action"]`` (so ``list``/``remove`` stay
+callable) but states the per-action requirement in each field's description
+and tightens the runtime error for ``add`` without ``message``.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from nanobot.agent.tools.cron import CronTool
+from nanobot.agent.tools.registry import ToolRegistry
+
+
+class _SvcStub:
+    """Minimal CronService stand-in; we only exercise schema/dispatch paths."""
+
+    def list_jobs(self):
+        return []
+
+    def get_job(self, _job_id):
+        return None
+
+    def remove_job(self, _job_id):
+        return "not-found"
+
+    def add_job(self, **kwargs):
+        class _J:
+            pass
+
+        j = _J()
+        j.id = "id1"
+        j.name = kwargs.get("name", "x")
+        return j
+
+
+@pytest.fixture
+def registry() -> ToolRegistry:
+    tool = CronTool(_SvcStub(), default_timezone="UTC")
+    tool.set_context("channel", "chat-id")
+    reg = ToolRegistry()
+    reg.register(tool)
+    return reg
+
+
+class TestSchemaContract:
+    def test_list_accepted_without_message(self, registry: ToolRegistry) -> None:
+        # action='list' must pass schema validation with nothing but 'action'.
+        _, _, err = registry.prepare_call("cron", {"action": "list"})
+        assert err is None
+
+    def test_remove_accepted_without_message(self, registry: ToolRegistry) -> None:
+        # action='remove' must pass schema validation with just 'action' + 'job_id'.
+        _, _, err = registry.prepare_call("cron", {"action": "remove", "job_id": "abc"})
+        assert err is None
+
+    def test_add_with_message_accepted(self, registry: ToolRegistry) -> None:
+        _, _, err = registry.prepare_call(
+            "cron", {"action": "add", "message": "ping", "at": "2030-01-01T00:00:00"}
+        )
+        assert err is None
+
+    def test_add_without_message_surfaces_actionable_runtime_error(
+        self, registry: ToolRegistry
+    ) -> None:
+        # Schema permits omitting message; the runtime must return a message
+        # that tells the LLM exactly what's missing and how to retry, so it
+        # doesn't loop like #3113 reports.
+        import asyncio
+
+        tool = registry._tools["cron"]  # type: ignore[attr-defined]
+        out = asyncio.run(tool.execute(action="add", at="2030-01-01T00:00:00"))
+        assert "message" in out
+        assert "add" in out
+        assert "Retry" in out or "retry" in out
+
+
+class TestSchemaSelfDescribesRequirements:
+    def test_message_description_flags_add_requirement(self) -> None:
+        # LLMs rely on field descriptions to infer when something is actually
+        # needed. Without this hint, #3113's loop returns.
+        tool = CronTool(_SvcStub())
+        desc = tool.parameters["properties"]["message"]["description"]
+        assert "REQUIRED" in desc and "action='add'" in desc
+
+    def test_job_id_description_flags_remove_requirement(self) -> None:
+        tool = CronTool(_SvcStub())
+        desc = tool.parameters["properties"]["job_id"]["description"]
+        assert "REQUIRED" in desc and "action='remove'" in desc
+
+    def test_top_level_required_stays_narrow(self) -> None:
+        # If 'message' or 'job_id' ever creep back into top-level required,
+        # list/remove start failing schema validation (the bug PR #3163 v1
+        # accidentally introduced).
+        tool = CronTool(_SvcStub())
+        assert tool.parameters["required"] == ["action"]