[Bug] --resume silently exits with 'no supported shell detected' on Windows, never opens interactive session

## Describe the bug

When using \copilot --resume=\ (or \--resume=\) on Windows, the CLI process starts and **immediately exits silently** without opening an interactive session and without printing any error to the terminal. The only evidence of failure is a tiny log file containing a single line.

## Error message

No output shown to the user. Log file (89 bytes) contains only:

\\\
2026-05-21T18:00:46.114Z [INFO] Shell completions: no supported shell detected, skipping
\\\

The process exits immediately after this line. Two separate resume attempts produced identical 89-byte log files — no session was loaded, no prompt appeared.

## Affected version

- **GitHub Copilot CLI 1.0.51** (Windows, installed via WinGet)
- OS: Windows 11
- Terminal: PowerShell 7 (pwsh)

## Steps to reproduce

1. Exit a named session with \/exit\ — CLI prints the resume hint, e.g. \copilot --resume="2026-05-21.1336"\
2. Run the printed command in a new terminal: \copilot --resume="2026-05-21.1336"\
3. Also attempted: \copilot --resume=\, \copilot --continue\
4. In all cases: process exits immediately, terminal returns to prompt, no interactive session opens

## Expected behavior

The interactive session resumes and the Copilot prompt appears.

## Actual behavior

The CLI exits silently. No error shown to user. Log shows only the shell completion detection line before process termination.

## Additional context

- The session itself was valid and loadable — successfully resumed via the in-app \/resume\ command from a separate running session
- The \--resume\ flag appears to exit before MCP servers are even initialized (contrast with successful resume logs which show MCP startup entries)
- Shell completion detection failing should be a non-fatal warning, not a reason to exit — especially when \--resume\ is explicitly passed

Start by reproducing --resume and --continue on Windows 11 in PowerShell 7, then inspect the 89-byte log and the shell-completion detection output. Trace startup around resume handling and verify that a valid session reaches the interactive prompt even when no supported shell is detected.

索引モデルが issue の本文から書いたものです。