feat: add halcon skill v2 plugin + fix marketplace source paths

- plugins/halcon: MVTec HALCON machine-vision skill v2 (local Windows HALCON
  + remote halcon-remote MCP). Bundles 2387-operator index, 20 official 24.11
  manuals, HDevEngine runners, ROI teaching, D2W overlay recipes.
- marketplace.json: register halcon; switch both entries to ./plugins/<name>
  source form (bare-name+pluginRoot was rejected by claude plugin validate
  v2.1.202); drop now-unused metadata.pluginRoot.

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

plugins/halcon/.claude-plugin/plugin.json

@@ -0,0 +1,8 @@
+{
+  "name": "halcon",
+  "description": "Drive MVTec HALCON machine vision — locally (Windows HALCON via WSL/uv/HDevEngine/hrun) or remotely over the halcon-remote MCP with no local install. Blob/edge/XLD, circle & shape fitting, 1D/2D/3D metrology, matching, calibration, bar/2D-code reading, OCR, D2W overlay. Bundles the 2387-operator index and 20 official 24.11 manuals.",
+  "version": "2.0.0",
+  "author": {
+    "name": "goldyard"
+  }
+}