fix(tmux): run set-option as separate commands on Windows for psmux compatibility

[john-mutuma]

Summary

On Windows with psmux, the start() method has two issues:

1. ; command chaining fails — PowerShell interprets ; as a statement separator and tries to run set-option as a standalone cmdlet.
2. Session names with spaces fail — self.id can contain spaces (e.g., "opencode 013d496a"), and psmux cannot handle spaces in -t arguments.

Fix

• Sanitizes session names — replaces spaces with hyphens for psmux compatibility, and syncs self.mux_session so other methods resolve correctly.
• Defers set-option calls — runs them as separate vim.fn.system() calls after a 1-second delay via vim.defer_fn, giving the session time to initialize.

The Linux/macOS code path is unchanged.

Mirrors upstream PR: folke#261