Add Codex PowerMem hook plugin

[wayyoungboy]

Summary

• add a first-party Codex hook plugin with setup, status, stop, and uninstall skills
• add Codex plugin packaging via Makefile and plugin release workflow artifacts
• package the Codex zip as a local marketplace root so the marketplace entry resolves after unzip
• keep CentOS 7 binary builds compatible with dependency source builds by enabling C99 and installing required image headers
• remove unrelated assets, deployment drafts, and scratch files from this PR

Validation

• go test ./... in apps/powermem-codex-plugin
• shell syntax checks for Codex plugin hook and lifecycle scripts
• JSON validation for marketplace, plugin manifest, and hook manifest
• Codex plugin manifest validation
• Codex plugin package script plus zip structure validation
• isolated Codex marketplace/install smoke from the packaged zip
• CentOS 7 C99 source-build smoke reproducing the old failure mode without CFLAGS and passing with CFLAGS=-std=c99

Notes

• GitHub Actions passed on the updated head, including Codex plugin packaging and native binary builds for CentOS 7 Linux amd64, macOS amd64/aarch64, and Windows amd64.