feat: add Cursor-compatible hooks and fix platform detection

Add hooks/hooks-cursor.json with Cursor's camelCase format (sessionStart,
version: 1) and update .cursor-plugin/plugin.json to reference it. Uses
${CURSOR_PLUGIN_ROOT} and run-hook.cmd for cross-platform support.

Fix session-start platform detection: check CURSOR_PLUGIN_ROOT first
(Cursor may also set CLAUDE_PLUGIN_ROOT), ensuring correct output format
for each platform.

Based on PR #709 with fixes for: wrong filename (.sh extension), missing
Windows support, fragile relative paths, and incorrect platform detection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.cursor-plugin/plugin.json

@@ -14,5 +14,5 @@
   "skills": "./skills/",
   "agents": "./agents/",
   "commands": "./commands/",
-  "hooks": "./hooks/hooks.json"
+  "hooks": "./hooks/hooks-cursor.json"
 }