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):