[Feature Request] Add keyboard shortcut (opt+m) for quick MCP server toggling

Problem

Currently, toggling MCP servers requires typing @ to trigger the mention UI, then selecting the server to enable/disable. This interrupts the prompt composition flow and requires multiple keystrokes.

Context

Recent versions introduced productivity-enhancing keyboard shortcuts that work independently of prompt text:

| Shortcut | Function | Version |
|----------|----------------------|---------|
| opt+p | Switch model | 2.0.65 |
| opt+t | Toggle thinking mode | 2.0.72 |

These shortcuts share a common pattern:

• Non-destructive: Don't modify the current prompt text
• Instant toggle: Single keystroke to change a session-wide setting
• Discoverable: Consistent modifier key (opt) for quick settings

MCP server toggling currently lacks this convenience despite being a frequently used operation during development sessions.

Proposed Solution

Add opt+m (or similar) to open an MCP server quick-toggle menu:

┌─ MCP Servers ────────────┐
│ ● typescript-lsp [enabled] │
│ ○ postgres [disabled] │
│ ● notion [enabled] │
│ ○ slack [disabled] │
└────────────────────┘

Behavior:

• Press opt+m → Opens server list overlay
• Use j/k or arrow keys to navigate
• Press Enter or Space to toggle selected server
• Press Esc to close
• Current prompt text remains untouched

Benefits

1. Consistency: Follows established opt+<key> pattern for quick settings
2. Efficiency: Single shortcut vs @ → search → select flow
3. Context preservation: No interference with prompt being composed
4. Discoverability: Users familiar with opt+p/opt+t will expect similar shortcuts

Alternatives Considered

| Alternative | Drawback |
|---------------------|--------------------------------------|
| Keep @ mention only | Requires prompt modification, slower |
| Add to /mcp command | Still requires typing, not instant |
| Use /config submenu | Too many navigation steps |

Additional Context

• Related: @-mention MCP toggle was added in 2.0.10
• Related: /mcp enable [server] and /mcp disable [server] added in 2.0.60
• The m key is mnemonic for "MCP" and currently unused in the opt+ namespace