fix: run SessionStart hook async to prevent Windows terminal freeze
The synchronous SessionStart hook blocked the TUI from entering raw
mode on Windows, freezing all keyboard input. The pure-bash
escape_for_json function is O(n^2) on Git Bash, taking 60+ seconds.
Running the hook async prevents the freeze while still injecting
superpowers context. Multiple users confirmed this workaround.
Fixes #404, #413, #414, #419