mara on !2910: "also remove imperative path for the things that are not
nodes yet, file follow up issue to fix that".
`TransientGuard`, the stored map and both manual set/clear are gone.
`transient_snapshot()` is derived and nothing else — destroy and
migration show no pill, because there is no node to derive one from. The
pill returns for free when they become nodes.
What those three guards were actually doing, though, was suppressing the
crash watcher, not drawing a pill. `migrate.rs` said so in its own
comment: without it, `crash_watch` fires `ContainerCrash` for every
migrated agent and the manager tries to recover containers that were
stopped on purpose. Destroy is the same — the container disappears
deliberately and nothing in the graph says so.
Deleting them outright would therefore have traded a dashboard pill for
false crash alerts on every destroy and every migration. So the
suppression survives as its own thing, `suppress_crash_watch`, with a
name that says what it is. It is still RAII, and still held for the
operation rather than stamped once, because the crash watcher's grace
window is finite and a destroy is not — a single tombstone would expire
mid-operation. The drop stamps the tombstone, covering the poll that
lands just after.
That leaves RAII in the codebase for exactly one purpose instead of two.
Untangling the pill from the suppression is what made the transient layer
deletable at all.
Follow-up issue for making destroy + migration real queue nodes to
follow; at that point this guard goes too.
Checked with clippy (`--all-targets -D warnings`), `cargo test -p
hive-c0re -p hive-jobq` (322 + 41 passed) and `nix fmt`.
7f920718f2