Extensions don't start for sessions created with create_session until the user switches to them #5380

Describe the bug

A session created with the session management tool create_session starts with no extension processes. Its model has none of the extensions' tools, none of the skills extensions register, and no extension hooks. Extensions start only when the user first switches to that session in the Sessions sidebar or with /resume, or when /restart restores it. Sessions the user starts directly, and prompt mode (copilot -p), load extensions within a second or two.

A created session is meant to work without the user watching, so it can spend minutes on its task without the tools it was asked to use.

Affected version

GitHub Copilot CLI 1.0.108-1 (also seen in 1.0.105, 1.0.106, and 1.0.107)

Steps to reproduce the behavior

1. Turn on session management tools (sessionManagementTools in ~/.copilot/settings.json).
2. Add a user extension, for example ~/.copilot/extensions/probe/extension.mjs, that calls joinSession({ tools: [{ name: "probe_ping", ... }] }).
3. In session A, ask the agent to call create_session with the prompt "Search your tools for probe_ping and say whether it exists." Stay in session A.
4. Session B reports that probe_ping doesn't exist. The process log has no extension launch for B's session ID, and B's events.jsonl has no extension events.
5. Switch to B in the sidebar. The log shows Registering foreground session: <B>, then Session changed (<A> → <B>), reloading extensions, followed by extension bootstrap lines for B. Send B any message; probe_ping is now available.

Expected behavior

A session created with create_session gets the same extensions as any interactive session from the moment it's created, since the host supervises it and it is meant to work without the user watching.

Additional context

• macOS on Apple silicon (arm64).
• Over two days on one machine, 10 sessions created by other sessions received their extensions between 36 seconds and 26 minutes after creation, each time when the user first opened the session.
• copilot-sdk/docs/extensions.md says joinSession() "attaches to the user's current foreground session", which may explain it.
• Possibly related: Switching sessions in the Sessions sidebar leaks plugin MCP servers and extension hosts (OOM) #5164 (switching sessions in the sidebar starts a set of extension hosts per session).