fix: remove invalid agents field from plugin.json

Claude Code auto-discovers the agents/ directory by convention.
The explicit "agents": "./agents" field fails manifest validation.
Official plugins (feature-dev, pr-review-toolkit) don't declare it.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rand

.claude-plugin/plugin.json

@@ -45,7 +45,6 @@
     "context-efficient"
   ],
   "skills": "./skills",
-  "agents": "./agents",
   "commands": "./commands",
   "hooks": {
     "SessionStart": [