hive-c0re: push HelperEvent::Spawned as a todo, not a broker message

Follow-up from #2955 (mara: 'make core able to give agent a todo').
First migration slice: Spawned was pure FYI-check-when-convenient
material, not something needing an immediate turn.

Coordinator::push_todo/push_todo_submitter do a best-effort live dial
of the target agent's own hive-agent-sock (hive_host_sock::agent_todo_
socket), sending the exact UpsertTodo request in-container producers
(matrix/bash/forge-notify) already send. Push, not queue: agent
offline (socket absent) or dial failure is a silent no-op, no retry,
no fallback delivery -- matches mara's 'not available if offline'
call exactly.

HelperEvent::Spawned removed entirely (enum variant + all 3 call
sites migrated: handle_spawn's two arms, finish_approval's Spawn
approval-kind arm) rather than kept alongside a translation layer --
per mara's correction on the first design attempt, migrating the
producer means deleting the old path, not bridging it.

Verified: cargo build/clippy/test -p hive-c0re -p hive-host-sock
-p hive-sh4re clean (318 tests), nix fmt clean.

Cargo.lock

@@ -1632,11 +1632,13 @@ dependencies = [
  "clap-markdown",
  "clap_complete",
  "forgejo-api",
+ "hive-agent-sock",
  "hive-core-agent-sock",
  "hive-host-sock",
  "hive-jobq",
  "hive-priv-sock",
  "hive-sh4re",
+ "hive-sock-client",
  "hive-types",
  "hmac 0.13.0",
  "indicatif",