#8811: fix(cron): handle atMs fallback for kind=at jobs

Fixes issue #8734. Tool-created cron jobs with `kind=at` were failing to fire because `nextRunAtMs` was missing in the job state. This was due to `computeJobNextRunAtMs` only checking `job.schedule.at` and not falling back to `atMs` (which the tool and CLI sometimes provide before normalization). Changes: - Updated `computeJobNextRunAtMs` to handle `atMs` fallback. - Improved `coerceSchedule` in normalization to better handle numeric `at` values and cleanup. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR fixes one-shot cron jobs (`schedule.kind="at"`) created by tooling that sometimes send `atMs` before normalization. It updates `computeJobNextRunAtMs` to fall back to `schedule.atMs` when `schedule.at` isn’t present, and it improves schedule normalization so numeric `schedule.at` values are coerced to an ISO timestamp and `atMs` is cleaned up when the job is determined to be an `at` schedule. Overall, the change fits into the cron subsystem’s existing split between input normalization (`src/cron/normalize.ts`) and runtime scheduling (`src/cron/service/jobs.ts` / `src/cron/schedule.ts`) by making the scheduler resilient to pre-normalized/legacy shapes coming from the CLI/tooling. <h3>Confidence Score: 3/5</h3> - This PR is likely safe to merge, but there’s a behavioral edge case around scheduling past `kind="at"` timestamps that’s worth confirming. - Changes are localized and align with the stated bug fix (fallback to `atMs` plus better normalization). The main remaining concern is `computeJobNextRunAtMs` now bypasses the `> nowMs` check used in `computeNextRunAtMs` for `kind="at"`, which could cause immediate execution for past timestamps (depending on intended semantics). Tooling/CI couldn’t be run in this environment (no node/pnpm), so confidence is moderated. - src/cron/service/jobs.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->