fix(kyberforge): keep .github/plugin/marketplace.json synced

apm.yml only declares claude and codex marketplace output profiles --
codex writes a differently-shaped file to .agents/plugins/marketplace.json,
so nothing regenerates the legacy Copilot CLI path at
.github/plugin/marketplace.json. It was hand-synced once during the #90
conversion and had drifted (missing the category field added when codex
output was enabled) with no gate to catch it.

scripts/sync-marketplace-mirror.sh keeps it byte-identical to the compiled
.claude-plugin/marketplace.json, wired as a pre-push check so it can't go
stale silently again.

Refs: #90

.github/plugin/marketplace.json

@@ -7,36 +7,42 @@
   },
   "plugins": [
     {
+      "category": "Developer Tools",
       "description": "Skills and agents for creating, maintaining, and managing a Claude Code / Copilot CLI plugin marketplace.",
       "name": "kyberforge",
       "source": "./plugins/kyberforge",
       "version": "1.4.0"
     },
     {
+      "category": "Utilities",
       "description": "A place for things to be binned",
       "name": "bin",
       "source": "./plugins/bin",
       "version": "1.1.1"
     },
     {
+      "category": "Version Control",
       "description": "Skills for working with Git \u2014 conventional commits, branch management, pull requests, and feature flow.",
       "name": "git",
       "source": "./plugins/git",
       "version": "1.3.2"
     },
     {
+      "category": "Version Control",
       "description": "Skills for managing Gitea repositories \u2014 issues, pull requests, milestones, releases, and wikis.",
       "name": "gitea",
       "source": "./plugins/gitea",
       "version": "1.3.3"
     },
     {
+      "category": "Productivity",
       "description": "Cross-cutting utility skills for everyday AI-assisted coding \u2014 triage, diagnosis, architecture review, and session navigation.",
       "name": "core",
       "source": "./plugins/core",
       "version": "1.1.0"
     },
     {
+      "category": "Productivity",
       "description": "Skills for Real Engineers \u2014 planning, TDD, architecture, and debugging workflows from Matt Pocock's .claude directory.",
       "name": "mattpocock-skills",
       "source": {
@@ -48,6 +54,7 @@
       }
     },
     {
+      "category": "Developer Tools",
       "description": "Skills and agents for configuring and running linters.",
       "name": "lint",
       "source": "./plugins/lint",