Add installable Codex and Agent Plugins packaging

[acoyfellow]

Summary

• add native Codex plugin packaging and marketplace metadata
• add portable Agent Plugins 1.0 manifests
• bundle the main Cloudflare Code Mode MCP server
• update installation guidance and MCP references
• keep the package current with the stable and @next Sandbox skill split

This carries forward Angus Hawkings's work in #93 on top of current main.

Codex install fix

The marketplace entry in #93 used a local ./ source for a plugin at the marketplace root. Codex accepted the marketplace but found no plugins:

No marketplace plugins found.
Error: plugin `cloudflare` was not found in marketplace `cloudflare`

This branch uses a Git-backed root plugin source. Codex can discover and install that source shape.

Validation

• parsed every JSON file with jq empty
• ran git diff --check main...HEAD
• reproduced the root-local-source failure with Codex CLI 0.150.1
• verified Codex CLI 0.150.1 discovers and installs the Git-backed root-plugin source shape