refactor(flake): add user registry and multi-user host support

Separate user identity (data) from the reusable modules, and let a host
declare any number of users instead of exactly one.

- users/registry.nix: per-user identity (name, email, groups, authorized
  and signing keys) as the single source of identity; no user data is
  hardcoded in the modules.
- mkHost takes a `users` set keyed by username; per-user identity is
  injected into each home config via the `identity` module arg
  (extraSpecialArgs is per-host, so it cannot carry per-user data).
- modules/users.nix builds accounts from the registry; modules/ssh.nix no
  longer defines authorized keys (the registry owns them); home/git.nix
  and home/desktop.nix read `identity`; users/emmathorpe/work.nix drops
  its now-redundant git identity override.
- Restructure the tree: users/, home/, modules/, hosts/, lib/ replace the
  former lyrathorpe/ and system/ layout.
- Add standalone homeConfigurations (the portable subset: shell, git,
  editor, claude) and an exported homeModules output for use on machines
  not managed by this flake, or as an input to other flakes.

Behaviour-preserving for existing hosts: lyrathorpe-mbp and
emmathorpe-edaas evaluate to identical derivations; lyrathorpe-t400,
lyrathorpe-macpro31 and lyrathorpe-rpi5 differ only by de-duplicating a
repeated authorized_keys entry. Fixes the SSH authorized-key leak (one
user's key was applied to every account), the hardcoded default git
identity, and the hardcoded EDaaS linger setting.

home/claude/memory/dev_clusters_disposable.md

@@ -0,0 +1,14 @@
+---
+name: dev-clusters-disposable
+description: "Lyra's dev clusters are disposable — break/mutate freely, no confirmation needed"
+metadata:
+  node_type: memory
+  type: feedback
+  originSessionId: 2b832088-ae38-4adf-b5bc-b5d44a8eac40
+---
+
+Dev clusters (e.g. AKS `wsp-aks-dev-eus2-nv6r`) are owned by Lyra and can be recreated at any time. Breaking them is always fine.
+
+**Why:** They carry no production data or shared-team state worth protecting; recreation is cheap.
+
+**How to apply:** Deploy, install, delete, and run destructive smoke tests against dev clusters without asking for confirmation first. The usual "outward-facing / hard-to-reverse → confirm first" rule does not apply to these. Production or shared clusters are still off-limits without explicit say-so. Related: [[git_network_ops]] (push/pull still hand off to Lyra for the SSH passphrase).