[Bug] --interactive startup prompt is silently dropped with a local plugin custom agent

Describe the bug

-i/--interactive <prompt> silently drops the startup prompt when Copilot CLI
is launched with a custom agent from local plugins. The TUI starts and the
requested agent is selected, but the prompt supplied on the command line is
never submitted. The session remains at zero turns.

This is using the standard GitHub Copilot provider, not a custom/BYOK provider.

Affected version

GitHub Copilot CLI 1.0.83-5
Windows 11, x86_64
Windows Terminal / PowerShell

Steps to reproduce the behavior

1. Load a local plugin that contributes a custom agent.

2. Start an interactive session with both the agent and an initial prompt:

   copilot `
     --plugin-dir "$HOME\git\bstack\bstack" `
     --plugin-dir "$HOME\git\bstack\workiq" `
     --agent bstack:Architect `
     --interactive "<ordinary prompt text>"
   

3. Observe that the TUI starts and selects bstack:Architect, but the supplied
   prompt is not executed.

4. Manually entering a prompt after startup works.

Expected behavior

The prompt passed to -i/--interactive should automatically execute as the
first turn, as documented by copilot --help:

-i, --interactive <prompt>
    Start interactive mode and automatically execute this prompt

Additional context

The debug log confirms that argument processing and custom-agent selection
succeeded:

"agent_name": "bstack:Architect"
"agent_display_name": "Architect"

However, the same process shut down with:

"turn_count": 0

There is no corresponding user.message, assistant turn start, or model call
for the command-line prompt. This rules out prompt interpretation or tool
execution as the failure point: the --interactive argument value never
becomes a user turn.

This resembles #4258, but that report was limited to custom/BYOK providers and
is closed. This reproduction uses the standard GitHub provider and a local
plugin custom agent on 1.0.83-5, so the failure is either a regression or a
broader unfixed startup-argument path.

#4592 may also be related because it documents custom-agent initialization
ordering differences between interactive and prompt modes.

Start by reproducing the documented -i/--interactive command with a local plugin custom agent, then trace CLI startup argument handling through agent selection. Done means the supplied prompt becomes the first user turn and executes automatically, with coverage for the custom-agent path.