fix: honor CLAUDE_CONFIG_DIR in hooks (#37)

ponytail-activate.js and ponytail-runtime.js hardcoded ~/.claude for the
flag file and settings lookup, ignoring CLAUDE_CONFIG_DIR. Add a shared
getClaudeDir() to ponytail-config.js and use it in both. Regression test
added to hooks.test.js.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

hooks/ponytail-runtime.js

@@ -1,11 +1,11 @@
 const fs = require('fs');
 const path = require('path');
-const os = require('os');
+const { getClaudeDir } = require('./ponytail-config');
 
 const isCodex = Boolean(process.env.PLUGIN_DATA);
 const statePath = isCodex
   ? path.join(process.env.PLUGIN_DATA, '.ponytail-active')
-  : path.join(os.homedir(), '.claude', '.ponytail-active');
+  : path.join(getClaudeDir(), '.ponytail-active');
 
 function setMode(mode) {
   fs.mkdirSync(path.dirname(statePath), { recursive: true });