Update Aegis

The skill checks the host-scoped registry first, can register new hosts with different sync modes (junction, copy-skills, plugin-managed), and won't clobber local changes without permission. It updates one host by default or all registered hosts if you pass `--all`, then verifies with aegis-doctor before reporting the commit delta and reload hint. The registry awareness is solid: if multiple hosts share the same method-pack root, it updates the pack once and refreshes each host's exposure separately. Useful if you're running Aegis across different AI coding environments and want predictable, evidence-first updates instead of background auto-update chaos.