Give plugins an agent's background work count and a background flag on hooks (#9362)

* feat(plugins): give plugins the agent's sidebar indicator and background flag

Plugin UI re-derived agent state from status and attention fields and could
not see background work, so it showed agents as done while a shell they left
running was still live. Agent snapshots now carry the sidebar's own bucket as
indicator/indicatorLabel, computed by the same function the sidebar uses so
the two cannot drift.

Lifecycle hooks and the agent.open.before request now carry `background`, so a
hook can skip a hidden helper agent without a lookup. A hook cannot change it;
an omitted value means unchanged, like metadata.

`finished` attention still means the turn finished; background work is the
indicator's concern.

* fix(plugins): keep session-open hooks that omit background typechecking

A hook that returns its own object instead of spreading the request has no
background field; the result type now treats it as optional, like metadata,
matching the runtime where an omitted value means unchanged.

* refactor(plugins): give agent snapshots the background work count instead of an indicator

* fix(plugins): keep legacy session-open hooks that omit background typechecking

plugin-examples/lifecycle-actions/README.md

@@ -2,7 +2,7 @@
 
 Small callbacks using the existing SDK:
 
-- Send `Try again.` when the latest output contains `out of credits`.
+- Send `Try again.` when the latest output contains `out of credits`, except to background agents.
 - Decline a shell request matching `rm -rf`; approve exactly `git status`; leave other requests alone.
 - Inject `PASEO_HOOK_CREATE_EXAMPLE=created` on creation and `PASEO_HOOK_OPEN_EXAMPLE=opened` on each session opening.
 - Change requested Codex agents to Claude Haiku before creation.