Feature Request: --disable-plugin flag for per-session plugin control

## Summary

Currently, there is no way to start a Copilot CLI session with specific plugins disabled without fully uninstalling them. A `--disable-plugin ` startup flag would allow lightweight sessions without losing plugin configuration.

## Current behaviour

The only way to start without a plugin is to uninstall it:
```bash
copilot plugin uninstall
# ...later...
copilot plugin install
```

## Proposed change

Add a `--disable-plugin ` flag (can be used multiple times, similar to the existing `--disable-mcp-server`):

```bash
# Start a minimal session — no user plugins, only built-in tools
copilot --disable-plugin security-best-practices --disable-plugin context-engineering

# Or a blanket flag to skip all plugins for this session
copilot --no-plugins
```

This mirrors the existing pattern of `--disable-mcp-server` and `--disable-builtin-mcps` which already provide per-session MCP control.

## Use case

Power users often configure multiple plugins for different workflows. A simple alias pattern like:

```bash
alias copilot_minimal='copilot --no-plugins --disable-mcp-server playwright --disable-mcp-server postgres'
```

...would let you switch between heavy and lightweight sessions without touching installed state.

## Related

- #2714 — requests persistent `plugin enable/disable` commands (different angle: that is config-level; this is per-session flag)

## Notes

The feature gap is noticeable because MCP servers already have this per-session control (`--disable-mcp-server`) but plugins do not. Parity between the two would make the CLI much more composable for scripting and alias-based workflows.

まず、既存の --disable-mcp-server オプションと --disable-builtin-mcps オプションの定義を見つけ、セッション起動時にプラグインがどのように読み込まれるかを追跡します。繰り返し指定された --disable-plugin フラグと --no-plugins の動作を定義し、無効化されたプラグインがインストール済みの設定を変更せずにスキップされることを示すテストカバレッジを追加します。

索引モデルが issue の本文から書いたものです。