feat(cron): serve cron run history from the task ledger

Cron executions now write full-fidelity task_runs rows (detail_json column,
store-key scoped) and cron.runs/CLI/task-maintenance read from the ledger;
legacy cron_run_logs dual-write stays as a revert safety net. Existing
history auto-imports at first state-DB open (verified against a production
DB copy: 2750/2750 rows, byte-identical parity, idempotent). Startup crash
recovery restores finished runs from finalized ledger rows instead of
reporting synthetic interruptions.

Part 1 of 2 for #106041.

src/cli/cron-cli/register.cron-simple.ts

@@ -64,7 +64,7 @@ async function waitForCronRunCompletion(params: {
   timeoutMs: number;
   pollIntervalMs: number;
 }): Promise<CronRunLogEntryResult> {
-  // Poll the run log rather than cron.run because completion state is written asynchronously.
+  // Poll the task ledger rather than cron.run because completion state is written asynchronously.
   const startedAt = Date.now();
   for (;;) {
     const page = (await callGatewayFromCli("cron.runs", params.opts, {