feat(kyberforge): enable codex marketplace output

Uncomment marketplace.outputs.codex in apm.yml. Codex output requires
every package to declare a category -- added one per package (Developer
Tools / Utilities / Version Control / Productivity, matching each
plugin's actual scope).

Re-running apm pack after enabling codex auto-synced 'codex' into each
local package's own targets: list (apm.yml, per plugin) -- packages
belonging to a marketplace that publishes a codex output need to declare
codex as a supported compile target for that to be coherent. Verified
idempotent: a second apm pack --check-clean run makes no further changes.

.claude-plugin/marketplace.json regenerated (now carries category per
plugin); .agents/plugins/marketplace.json is the new codex artifact.

Refs: #90

.claude-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",