fix(cronjob): advertise 'custom:<name>' provider format in tool schema

The `provider` field in CRONJOB_SCHEMA only showed examples like
'openrouter' and 'anthropic', with no mention of the canonical
'custom:<name>' form required for custom_providers entries. When the
user has custom providers configured, LLMs tend to write the bare type
name ('custom') because the schema does not advertise the ':<name>'
suffix. The bare value then serializes into jobs.json and causes the
cron job to fail silently at run time — `_resolve_model_override`
treats it as a user-specified provider and skips the pin-to-current
fallback, but no provider ever resolves from the bare 'custom' string.

Clarifying the schema so the canonical form is discoverable addresses
the root cause at the tool-definition boundary.

tools/cronjob_tools.py

@@ -513,7 +513,7 @@ Important safety rule: cron-run sessions should not recursively schedule more cr
                 "properties": {
                     "provider": {
                         "type": "string",
-                        "description": "Provider name (e.g. 'openrouter', 'anthropic'). Omit to use and pin the current provider."
+                        "description": "Provider name (e.g. 'openrouter', 'anthropic', or 'custom:<name>' for a provider defined in custom_providers config — always include the ':<name>' suffix, never pass the bare 'custom'). Omit to use and pin the current provider."
                     },
                     "model": {
                         "type": "string",