Migrate live secrets to sops-nix (Milestone 2)

Audited the working tree and full git history for committed secrets
(gitleaks + trufflehog + manual grep, see secrets-inventory.md, kept
local/gitignored per the spec). Found: a password hash shared by root
and the nixos user across every host, two live Beszel monitoring
tokens, and a GitHub fine-grained PAT embedded in a home-manager
nix.conf.

Migrates all of them to sops-nix:
- .sops.yaml + secrets/*.yaml, encrypted for admin + the age keys
  derived (via ssh-to-age) from each live host's existing SSH host
  key — no new key material transferred to any machine.
- users.users.{root,nixos}.hashedPasswordFile replaces the inline
  hashedPassword shared by every target.
- The GitHub PAT moves from a home-manager-managed, store-visible
  nix.conf to a sops.templates-rendered file included via nix.conf's
  native !include, system-wide instead of per-user.
- Beszel TOKEN moves from `environment` (store-visible) to
  `environmentFile` (runtime-only via sops.templates); the dead
  commented-out docker token is removed from the tree entirely.

Added a tracked pre-commit hook (gitleaks protect --staged, wired via
core.hooksPath) so a secret can't be committed by accident again, and
documented the sops workflow in README.md.

Structural verification only: all 17 flake targets evaluate, and
`nix build --dry-run --no-link` succeeds for the three currently
deployed hosts. Per CLAUDE.md, actual `nixos-rebuild switch` — the
step that confirms secrets decrypt and services start on a real
machine — is left for manual verification.

Git history still contains the original plaintext secrets; scrubbing
history (Milestone 3) and rotating every credential (Milestone 4) are
separate, deliberately gated steps per remove-sensetive-info-refactor.md.

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

README.md

@@ -86,7 +86,25 @@ review sessions.
 
 ## Security Notes
 
-Do not commit tokens, private keys, live credentials, or new password hashes.
-This repository currently contains committed password hashes in shared NixOS
-configuration; rotate those passwords and move hashes into host-local secret
-management before treating the repository as public or widely shared.
+Do not commit tokens, private keys, live credentials, or new password hashes
+as plaintext. Secrets are managed with [sops-nix](https://github.com/Mic92/sops-nix):
+encrypted files live under `secrets/`, recipients (per-host age keys derived
+from each host's existing SSH host key, plus an admin key) are declared in
+`.sops.yaml`. To add or edit a secret:
+
+```bash
+nix-shell -p sops --run "sops secrets/<file>.yaml"
+```
+
+then reference it from a module via `config.sops.secrets."<name>".path`
+(or `sops.templates` for values that need to be embedded in a rendered
+config file, e.g. `nix.conf`'s `access-tokens`). Never write a secret value
+directly into a tracked `.nix` file. A pre-commit hook (`.githooks/`,
+enabled via `git config core.hooksPath .githooks`, done automatically by
+`scripts/codex-setup.sh`) runs `gitleaks protect --staged` to catch mistakes
+before they're committed.
+
+This repository's git *history* still contains secrets committed before this
+migration (see `remove-sensetive-info-refactor.md`) — those are being
+scrubbed and rotated separately; don't treat the repo as safe to make public
+until that's finished.