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/agent-configuration/README.md

@@ -6,7 +6,7 @@ Sets saved Codex options through `server.addHook("agent.create.before", async ..
 - Sets `config.providerOptions.approval_policy` to `on-request`.
 - Preserves other configuration and provider options.
 
-Adds the `company` MCP server to Codex launches through
+Adds the `company` MCP server to Codex launches, skipping background agents, through
 `server.addHook("agent.open.before", async ...)`. The server is never saved in the agent's configuration;
 existing saved MCP servers are preserved. Existing agents receive the server through the
 [session reopen behavior](../../public-docs/plugins/hooks.md#attach-tools-to-agents).

plugin-examples/agent-configuration/index.server.ts

@@ -20,7 +20,8 @@ export default function contribute(server: PluginServerContext) {
   });
 
   server.addHook("agent.open.before", async ({ request }) => {
-    if (request.provider !== "codex") {
+    // Background agents are short-lived helpers; keep the company tools for the agents users open.
+    if (request.provider !== "codex" || request.background) {
       return;
     }