Two hardening items from argus's review of #2374:
1. PR state check at submission:
- Add `pr_is_open(repo, pr)` to forge/pr_merge.rs using
`repo_get_pull_request` + `StateType` — early error if the PR is
already closed or merged instead of queuing a card that fails later
- Call it in `submit_merge_config_pr` before fetching the head sha
2. Atomic fetched_sha INSERT:
- Add `fetched_sha: Option<&str>` to `Approvals::submit_kind` so
the sha can be included in the INSERT rather than a follow-up UPDATE
- MergeConfigPr already knows the sha before inserting the row
(pr_head_sha runs first) → pass `Some(&sha)`, drop the separate
`set_fetched_sha` call → truly atomic
- ApplyCommit still needs two writes (sha resolved by git_fetch_to_tag
after the row exists) → pass `None`, `set_fetched_sha` unchanged
- All other callers (InitConfig, Spawn, UpdateMetaInputs,
SchedulePrompt) pass `None` — no behavioural change
- Add `fetched_sha_in_insert_is_readable_via_get` test covering the
MergeConfigPr path
