feat(codex): install codebase-memory skill

[agn-7]

Summary

• install the existing consolidated codebase-memory Skill for Codex users under HOME/.agents/skills/codebase-memory/SKILL.md
• preserve .codex/AGENTS.md as the lightweight always-loaded guidance while using the Skill for progressive-disclosure workflows
• include the Codex skill path in install --plan and remove the managed skill during uninstall
• update smoke/security install checks and the README multi-agent table

Closes #976

Testing

• ./build/c/test-runner cli
• make -f Makefile.cbm cbm
• bash -n scripts/security-install.sh
• bash -n scripts/smoke-test.sh
• fake HOME install --plan check for .agents/skills without mutation
• fake HOME install/uninstall cycle verifying Codex skill write/remove and existing config preservation
• bash scripts/security-install.sh build/c/codebase-memory-mcp

Note: make -f Makefile.cbm test currently exits nonzero in existing graph/edge regression suites with no graph DB; the touched CLI suite passes.

[DeusData]

Thanks for the PR. I have triaged this with #976 into 0.9.2-rc as a Codex integration enhancement. The review focus will be install/uninstall ownership boundaries, preserving existing Codex config, and making sure managed skill files are clearly scoped to codebase-memory-mcp.