Add cron action executor (scheduled trigger)

[jaredzwick]

Summary

• Second action executor for pypesdev/agents. A cron action wraps another Action with a 5-, 6-, or 7-field cron expression and fires it on schedule via an in-process scheduler. Mirrors the typed-spec pattern of the webhook executor shipped in Add webhook action executor (HTTP POST) #8.
• src/executors/cron.rs exposes a pure parse / next_fire_after API plus a Scheduler with a tick(now) shape so unit tests drive it with a mock clock and the real loop drives it with Utc::now().
• process_actions reports the next computed fire time for cron entries (ExecutionOutcome::Cron(...)); actual firing is handled by Scheduler. Out of scope for v1: distributed scheduling and persistent missed-fire catchup.

Example output

```
$ cargo run --example cron_executor
→ stored action: {"type":"cron","expression":"* * * * * *","action":{"type":"webhook","url":"http://127.0.0.1:51657/hook\",\"headers\":{},\"payload\":{\"event\":\"cron.tick\",\"n\":1}}}
⏲ next fire scheduled at 2026-05-03 09:11:52 UTC (in 943 ms)
← cron[0] fired webhook → status=200 body={"ok":true}
✓ mock receiver got 1 request(s):
{"event":"cron.tick","n":1}
```

Test plan

• cargo test — 18 passed (10 new, 8 pre-existing)
• cargo run --example cron_executor — terminates in <1s, fires the wrapped webhook, mock receiver captures the payload
• Mock-clock unit test (scheduler_fires_on_time_with_mock_clock) drives due_at / advance deterministically
• Real-time integration test (scheduler_next_due_waits_for_real_tick) confirms next_due blocks until the scheduled instant
• Round-trip serde test for the recursive Action::Cron { action: Box<Action> } shape

Closes HIR-118.

[jaredzwick]

CTO review — approved, merging

Verified:

• 18/18 tests green; new tests cover 5/6/7-field parse, strictly-after semantics, mock-clock fire-on-time, real-clock integration, and Action::Cron serde round-trip.
• Pure parse / next_fire_after split from stateful Scheduler — testable and cleanly composable.
• process_actions correctly reports next fire without firing inline; firing routes through Scheduler.
• Example terminates in ~1s as specified; README + CHANGELOG updated.
• Out-of-scope items (distributed scheduling, restart catchup, LLM executor) match the ticket.

Follow-up (separate ticket I'll file): wire Scheduler into a long-running daemon runtime so cron actions actually fire in production. Daemon today only handles process plumbing — needs an agent execution loop. Not blocking this merge.