fix: use CLAUDE_PLUGIN_ROOT in hooks.json, drop duplicate manifest hooks

hooks/hooks.json used ${PLUGIN_ROOT}, which Claude Code never defines, so the literal resolved against the hook process cwd and SessionStart/UserPromptSubmit failed with 'Cannot find module'. Switch to ${CLAUDE_PLUGIN_ROOT} (and %CLAUDE_PLUGIN_ROOT% for the Codex commandWindows variant); Codex aliases CLAUDE_PLUGIN_ROOT so both hosts resolve. Drop the duplicate inline hooks block from .claude-plugin/plugin.json so hooks load from a single canonical source.

.claude-plugin/plugin.json

@@ -5,31 +5,5 @@
   "author": {
     "name": "Dietrich Gebert",
     "url": "https://github.com/DietrichGebert"
-  },
-  "hooks": {
-    "SessionStart": [
-      {
-        "hooks": [
-          {
-            "type": "command",
-            "command": "node ${CLAUDE_PLUGIN_ROOT}/hooks/ponytail-activate.js",
-            "timeout": 5,
-            "statusMessage": "Loading ponytail mode..."
-          }
-        ]
-      }
-    ],
-    "UserPromptSubmit": [
-      {
-        "hooks": [
-          {
-            "type": "command",
-            "command": "node ${CLAUDE_PLUGIN_ROOT}/hooks/ponytail-mode-tracker.js",
-            "timeout": 5,
-            "statusMessage": "Tracking ponytail mode..."
-          }
-        ]
-      }
-    ]
   }
 }