fix(cron): replace pending schedule when its expression changes

[psteinroe]

Changing a static cron expression now replaces the pending occurrence. Previously the old schedule kept running next to the new one. The PR also documents the "one queue = one codebase" rule that worker registration relies on.

Changed cron expressions

Registration used to match a stale static schedule by (task_key, schedule name) only. Changing 0 0 9 * * * to 0 0 10 * * * under the same name therefore left both the old and the new scheduled::daily::<ts> chains pending forever. _private_register_worker now matches by (task_key, schedule name, cron_expression). Pending old-expression occurrences are deleted, and in-flight ones are cancelled, the same way removed schedules already were. Overdue occurrences of an unchanged schedule are still kept across restarts.

One queue = one codebase

Every orchestrator polling a queue runs the same task set, and separate services use separate queues, the same constraint as Temporal task queues. Registration therefore owns the whole queue, and queue-wide cleanup is intended: it removes schedules and subscriptions of tasks deleted from the codebase.

• The rule is documented in docs/content/scaling/horizontal.md, with short pointers from the concurrency and cron docs and from AGENTS.md.
• Known edge case, also documented: during a rolling deploy, an old replica that restarts re-registers the old task set. That removes the cron schedules and event subscriptions that only the new version declares, until a new-version replica starts again. Registration only runs at worker start, so if this happens after the rollout finished, restart one new replica.

Verification

• New integration test changing a cron expression replaces the pending schedule in cron-scheduling.test.ts. It fails before the fix, and the coordinator's R2 repro passes with it.
• just lint, just format, bun run typecheck and the full bun test (342 pass) pass on top of current main.