--agent <name> selection is dropped ~1s after startup (race with sign-in / MCP reload) #5124

Describe the bug

When starting the CLI with --agent <name>, the custom agent is selected and then silently deselected ~1 s later, before any user input. The banner shows Selected custom agent: my-agent, then MCP Servers reloaded: 1 server connected, and the agent disappears from the status bar; the session falls back to the default agent.

It looks like a race between the startup agent selection and the agent/MCP refresh after sign-in. events.jsonl of an affected session (no user input in between):

14:59:11.603 subagent.selected {"agentName":"my-agent"}
14:59:11.765 subagent.selected {"agentName":"my-agent"}
14:59:11.717 subagent.selected {"agentName":"my-agent"}   <- out of order, concurrent re-selection
14:59:12.400 subagent.deselected {}

The deselect follows Signed in to https://github.com and the MCP client re-init in the process log (Service initialized as client -> task cancelled -> Service initialized as client).

Affected version

1.0.95

Steps to reproduce the behavior

1. Have a custom agent available (user-level ~/.copilot/agents/my-agent.md, or plugin-provided).
2. Start interactively: copilot --agent my-agent
3. Don't type anything; watch the banner and status bar for ~2 s.
4. Agent is gone from the status bar; events.jsonl shows subagent.deselected.

Expected behavior

The agent passed via --agent stays selected for the whole session.

Additional context

• Frequency (automated pty launches, no input, 15 s each): dropped 5/6 with a user-level agent (--agent my-agent), 4/6 with a plugin agent (--agent myplugin:my-agent).
• When MCP startup failed (Failed to start MCP Servers: session not found), the agent was never dropped, which points to the MCP reload / agent-list refresh path clearing the selection.
• Workaround: run /agent my-agent after startup.
• macOS (Apple Silicon, arm64), zsh, launched via shell alias with --allow-all and several --deny-tool flags.