refactor(halcon): single-skill plugin layout → invoke as /halcon (not /halcon:halcon)

Move SKILL.md + references/ scripts/ evals/ from skills/halcon/ up to the
plugin root. Per Claude Code plugins-reference, a plugin with SKILL.md at its
root and no skills/ subdir is auto-loaded as a single-skill plugin (v2.1.142+),
so the invocation name = frontmatter name = halcon → clean /halcon.
Bump plugin.json 2.0.0 → 2.0.1 so existing installs receive the update.

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

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

@@ -1,7 +1,7 @@
 {
   "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",
+  "version": "2.0.1",
   "author": {
     "name": "goldyard"
   }