Convert Mercury into a persistent development host

[rrvsh]

Summary

• keep ~/Agents/worktrees machine-local instead of synchronizing it across development hosts
• retire Hermes from Mercury while preserving the shared development profile and Pi session sync
• add Mercury to the cross-host ZMX picker
• persist Mercury deployments in the NixOS system profile before switching
• remove the unused Hermes input, module, cleanup tooling, and encrypted environment file

Validation

• nix develop -c just format-nix
• nix develop -c just check-nix
• git diff --check
• built Mercury’s NixOS system on Nemesis and deployed it with nix copy
• rebuilt and activated Alpha and Kikir from the branch
• activated the Nemesis Home Manager generation
• verified all four active Fish configs include Mercury in the ZMX host list
• verified Syncthing is active on all four hosts with connected peers and healthy listeners
• verified a probe under ~/Agents/worktrees did not synchronize to any peer
• verified Mercury’s running system matches its persistent system profile
• verified Mercury has no Hermes units and no failed system or user units