Add a new configuration option `exec_timeout_minutes` under the `tools.cron`
section to control the maximum execution time for cron jobs. The default
timeout is set to 5 minutes, which is appropriate for LLM operations.
The configuration can be set in the config file or via the
`PICOCLAW_TOOLS_CRON_EXEC_TIMEOUT_MINUTES` environment variable. A value of
0 disables the timeout entirely.
This change improves system reliability by preventing cron jobs from running
indefinitely in case of unexpected failures or hanging processes.
