Add OSC 9;9 support for PowerShell prompt to enable Windows Terminal directory tracking

[Copilot]

Implements OSC 9;9 escape sequence emission in Cmder's PowerShell prompt to enable Windows Terminal's "Duplicate Tab" and "Split Pane" features to preserve the current working directory. This matches the recent fix for cmd.exe/Clink and follows Microsoft's published guidance for Windows Terminal integration.

Changes Made

Modified vendor/profile.ps1:

• Added OSC 9;9 sequence emission in the main $Prompt scriptblock
• Only emits when running in Windows Terminal ($env:WT_SESSION) or ConEmu ($env:ConEmuPID)
• Only emits for FileSystem provider paths (skips Registry, etc.)
• Added comprehensive comments explaining the functionality

Updated README.md:

• Added documentation about Windows Terminal directory tracking support
• Noted that both cmd.exe and PowerShell sessions now support this functionality

Implementation Details

The OSC 9;9 sequence (ESC]9;9;"<path>"ESC\) is emitted before each prompt display when:

1. Running in a supported terminal environment (Windows Terminal or ConEmu)
2. Current location uses the FileSystem provider
3. All conditions are met for directory tracking

This implementation is:

• Non-invasive: No changes to prompt appearance or existing functionality
• Compatible: Works with existing posh-git and user customizations
• Gated: Only active in supported terminal environments
• Safe: Only emits for appropriate path types

Example Usage

When using Cmder with PowerShell in Windows Terminal:

1. Navigate to any directory: cd C:\Projects\MyApp
2. Use Ctrl+Shift+D to duplicate tab → new tab opens in C:\Projects\MyApp
3. Use split pane operations → new panes open in the same directory

This provides the same seamless directory tracking experience that cmd.exe/Clink users have enjoyed.

Fixes #3025.

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