Closes the #3110 split — lib.rs is now just the crate doc comment and
the pub mod list.
journal.rs's new doc comment fixes a pre-existing bug: the old
JournalPriority doc text in lib.rs was actually half Capability's doc
(a leftover from an earlier reorder that moved the code but not the
comment above it).
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
