refactor(modules): extract scheduling as registry-based module

Moves the scheduling surface — 5 delivery actions (schedule_task,
cancel_task, pause_task, resume_task, update_task), handleRecurrence,
applyPreTaskScripts, and task DB helpers — out of core and into
src/modules/scheduling/ (host) and container/agent-runner/src/scheduling/
(container).

First PR to fill the MODULE-HOOK markers introduced in PR #2:
  - src/host-sweep.ts MODULE-HOOK:scheduling-recurrence now dynamically
    imports handleRecurrence from the module each sweep tick.
  - container/agent-runner/src/poll-loop.ts MODULE-HOOK:scheduling-pre-task
    dynamically imports applyPreTaskScripts before the provider call.
    When the marker block is empty (scheduling uninstalled), `keep`
    falls back to `normalMessages` so non-task messages still flow.

The 5 task cases are removed from delivery.ts's handleSystemAction
switch — the registry now routes them. Task DB helpers moved out of
src/db/session-db.ts (which kept `nextEvenSeq` as a named export so
the module can uphold the host-writes-even-seq invariant). Test suite
split to match: scheduling-specific tests live in the module.

No migration — tasks are messages_in rows with kind='task'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/modules/index.ts

@@ -15,4 +15,5 @@
  */
 import './interactive/index.js';
 import './approvals/index.js';
+import './scheduling/index.js';