Deduplicate reusable shell code in scripts/ into scripts/lib/

Three chunks of copy-pasted logic were drifting across scripts/*.sh:
- codex-setup.sh and codex-maintenance.sh each carried an identical
  NIX_CONFIG bootstrap + ensure_nix_profile() -> scripts/lib/nix-bootstrap.sh
- sync-host-keys.sh and prepare-host-key.sh each ran the same
  ssh-keygen/ssh-to-age nix-shell invocations -> scripts/lib/ssh-host-keys.sh
  (prepare-host-key.sh now also calls env.sh's nix_extra_opts before using
  them, closing a gap where it alone skipped the nix-cache reachability
  check env.sh exists for)
- the "list nixosConfigurations attrNames" / "get one target's hostName"
  nix eval pattern was repeated across codex-setup.sh, codex-maintenance.sh,
  sync-host-keys.sh and create-proxmox-resource.sh (the latter twice, in
  its own --list and --host lookup) -> scripts/lib/nix-eval.sh, which also
  centralizes the --no-use-registries --no-accept-flake-config flag pair
  used on every such call

Verified against the real flake/node config (nix is available here):
create-proxmox-resource.sh --list for both --type lxc/vm, a full
--dry-run create, and prepare-host-key.sh generating and cleaning up a
real key/age-pubkey pair.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/lib/nix-bootstrap.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Shared Nix bootstrap for scripts/codex-setup.sh and
+# scripts/codex-maintenance.sh: the nix.conf settings both need in effect
+# before a single `nix` command runs (flakes enabled, never honor a flake
+# input's own nixConfig, no "dirty tree" warning spam), plus a helper to
+# pull an already-installed Nix's daemon/profile script onto PATH if it
+# isn't there yet. Source this instead of copying it -- see CLAUDE.md.
+export NIX_CONFIG="${NIX_CONFIG:-}
+experimental-features = nix-command flakes
+accept-flake-config = false
+warn-dirty = false
+"
+
+ensure_nix_profile() {
+  if [ -f /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh ]; then
+    . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
+  elif [ -f "$HOME/.nix-profile/etc/profile.d/nix.sh" ]; then
+    . "$HOME/.nix-profile/etc/profile.d/nix.sh"
+  fi
+}