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

hooks/hooks.json

@@ -6,7 +6,8 @@
         "hooks": [
           {
             "type": "command",
-            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh"
+            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/session-start.sh",
+            "async": true
           }
         ]
       }