feat(secrets): decrypt the login password from sops (task 0010)

Wire sops-nix into the shared base config as unconditional plumbing, with
a two-tier age identity model: an admin identity held outside the repo, and
a per-host identity generated on the machine and kept on its encrypted root.
Both `sshKeyPaths` defaults are cleared so the SSH host keys stay out of the
decryption path and remain free to become secrets in their own right.

The primary user's password hash moves into a shared secrets file encrypted
to admin plus neogaia, consumed through `hashedPasswordFile` and decrypted
before accounts are created.

This needs `users.mutableUsers = false`: NixOS applies a declared hash to an
already-existing account only when that flag is false, so at the default the
hand-set password would have been kept and the change would have been inert.
Root consequently has no password and is locked; sudo from wheel is the way
in, and generation rollback remains the recovery path.

CLAUDE.md

@@ -15,6 +15,11 @@ The domain model (Host, Module, Skeleton, Auto-loader, Enable convention, overla
   A file-top header is one concise purpose line, added only where the filename or path does not already say it — never a feature inventory of the code below.
   For a placeholder, say so plainly plus any actionable present-tense directive ("Placeholder: regenerate with nixos-generate-config on the target machine"), never "placeholder for <missing feature>".
   Option `description`/`mkEnableOption` strings are user-facing documentation rather than comments, so they may describe behaviour more fully — but the self-contained rule and the bans on glossary terms and agent-state references still apply.
+- Break comment lines early, at sentence and clause boundaries, rather than greedy-wrapping at the right margin.
+  Start each sentence on its own line, and when one sentence needs several lines, break it at a comma or semicolon rather than mid-phrase.
+  A line that ends mid-noun-phrase ("keeps the SSH host keys out / of the decryption path") is the smell.
+  This is the Markdown one-sentence-per-line rule applied to code comments; lines may end well short of the margin.
+  Only reformat comments you are actually writing or changing.
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