fix: resolve packaged MCP resources from plugin root

[viragtripathi]

Summary

• run the Toolbox stdio server from the installed plugin root and load its bundled tools.yaml
• forward CockroachDB connection variables with env_vars and source the Cloud cluster header with env_http_headers
• document stdio versus independently started HTTP behavior and remove the obsolete absolute cache-path workaround
• add relocation-focused package tests and gate pull requests and releases on them

Verification

• python3 -m unittest -v tests/test_plugin_package.py
• clean marketplace clone: all synced skills present as real files; manifest, MCP config, hooks, and tools.yaml included
• Codex cachebuster reinstall: installed .mcp.json and tools.yaml matched source; config resolved inside the installed cache directory
• release archive inspection: .mcp.json, tools.yaml, and plugin.json included
• skills-only OpenAI submission bundle dry run: MCP resources remain excluded as intended
• git diff --check

Notes

The plugin version is intentionally unchanged. Release Please owns the version and will propose the patch release after this fix reaches main.

The user-level Codex MCP override and ~/.codex/config.toml were not modified.