Add dangerous command blocking and cron agent scheduling

Dangerous command approval: run_command skill now checks commands
against 9 regex patterns (rm -rf /, dd, mkfs, fork bombs, shutdown,
device writes, etc.) and blocks execution with a clear message.
Defense-in-depth layer on top of VM isolation.

Cron agents: templates support schedule (5-field cron) and
schedule_timeout (seconds, default 300) fields. Overseer checks
every 60s, spawns {name}-cron agents on match, auto-destroys after
timeout. Inline cron parser supports *, ranges, lists, and steps.
No npm dependencies added.

TODO.md

@@ -30,8 +30,8 @@
 
 ### Bigger items
 - [ ] Skill learning — agents create new skills from experience
-- [ ] Cron agents — scheduled agent spawns
-- [ ] Dangerous command approval — pattern detection + allowlist
+- [x] Cron agents — scheduled agent spawns (5-field cron in templates, auto-destroy timeout)
+- [x] Dangerous command approval — pattern detection blocks rm -rf /, dd, mkfs, fork bombs, etc.
 - [ ] Parallel tool execution — concurrent independent tool calls
 
 ## Polish