[FEATURE REQUEST] Add slash commands for switching permission modes within session

Problem

Currently, switching between permission modes (default/auto-accept/plan) requires either:

1. Restarting the entire Claude Code session with --permission-mode plan
2. Using Shift+Tab keyboard shortcut (which is broken on Windows - see issues #3390, #3464)

This creates poor UX, especially when the keyboard shortcut doesn't work.

## Proposed Solution
Add slash commands to switch permission modes within an active session:

• /mode default - Switch to default permission mode
• /mode auto-accept - Switch to auto-accept edit mode
• /mode plan - Switch to plan mode
• /mode - Display current permission mode

## Benefits

• No need to restart sessions
• Workaround for broken Shift+Tab on Windows
• More discoverable than keyboard shortcuts
• Consistent with existing slash command pattern

## Use Case
Developer wants to switch from plan mode to auto-accept mode after reviewing a plan, without losing conversation
context by restarting the session.