Add Codex plugin marketplace support

[robobryce]

Summary

• Add a repo-level .agents/plugins/marketplace.json so Codex and agent marketplace consumers can discover the agitentic plugin.
• Add plugins/agitentic/.codex-plugin/plugin.json with skills: ./skills/ and presentation metadata.
• Keep the existing Claude marketplace intact and update docs/tests to cover both manifest families.

Test plan

• python3 /home/gateroom/.agents/skills/.system/plugin-creator/scripts/validate_plugin.py plugins/agitentic.
  • Plugin validation passed: /home/gateroom/agitentic-fix/plugins/agitentic
• ./test.bash.
  • Skill validation: validated 4 skill(s).
  • BATS summary: ok 77 git-sync --prune deletes merged branches, keeps novel / current / synced.
• ./test.bash --secrets.
  • 36 commits scanned.
  • no leaks found
  • scan completed in 14.7ms
  • no leaks found
• Docker live install in a fresh node:20-bookworm-slim container with Codex and Claude Code installed from npm.
  • codex: codex-cli 0.133.0
  • claude: 2.1.148 (Claude Code)
  • Added plugin 'agitentic' from marketplace 'robobryce-agitentic'.
  • agitentic@robobryce-agitentic installed, enabled 0.3.0
  • Successfully installed plugin: agitentic@robobryce-agitentic (scope: user)