chore(skills): add /sessions skill (vendored from EveryInc compound-engineering ce-sessions)

Searches Claude Code / Codex / Cursor session history and synthesizes
what was tried, decided, and learned in prior sessions. Real files in
.agents/skills/sessions, symlinked into .claude/skills/ to match the
existing skill convention. Recovered the already-known root cause of the
attachment EPROTO failure (transient network TLS interference, not infra).

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>