feat(api): add CronClient for /crontab endpoints

[Copilot]

Adds CronClient to cover the three previously unwrapped /crontab/{site}/* endpoints, modelled on edge_cache.py.

New: atomic_sdk/api/cron.py

• list(site_id|domain) → GET /crontab/{site}/list
• add(schedule, command, site_id|domain) → POST /crontab/{site}/add; raises ValueError on empty schedule or command
• remove(cron_id, site_id|domain) → POST /crontab/{site}/remove
• find(cron_id=…|command=…, site_id|domain) — client-side filter over list()
• clear(site_id|domain) — destructive list+remove-all; returns removed IDs

All methods raise ValueError when neither site_id nor domain is provided. 4xx/5xx errors surface via inherited _request translation — no bespoke try/except.

Schedule semantics and platform constraints (8 h max runtime, no stacking, 3-concurrent limit, site-cron-results webhook on failure) are documented in docstrings.

atomic_sdk/client.py

CronClient imported and attached as self.cron, alphabetically between client and custom_certificates.

examples/cron/01_manage_cron.py

entries = client.cron.list(domain="example.com")
result  = client.cron.add(schedule="hourly", command="wp cron event run --due-now", domain="example.com")
entry   = client.cron.find(cron_id=result["cron_id"], domain="example.com")
client.cron.remove(cron_id=result["cron_id"], domain="example.com")

Both examples/README.md and the top-level README.md updated to include Cron in the supported-resources list.