Isolate backup credentials and pin the assistant runtime

[assistant-ngalaiko]

Changes

• Strip documented B2/restic variables from service processes, loggers, and hooks. Only backup restore and cron opt in.
• Restrict s6's environment directory to root on boot and activation.
• Load Amp's user-owned configuration after dropping privileges.
• Lock ngalaiko/assistant; provision its tools, skills, browser, fonts, and runtime launchers.
• Seed global runtime instructions and skills without replacing custom files. Preserve editable checkouts; initialize new ones at the locked revision.
• Keep credentials, account configuration, and private overrides outside Git and the Nix store.

Validation

• Removed the added runtime and credential tests at the user's request; the existing formatting check remains.
• nix flake check --all-systems --no-update-lock-file passed after removal (all platforms evaluated, formatting executed on x86-64 Linux).
• x86-64 system generation built successfully after removal.
• Full x86-64 OCI image built successfully before the test-only removal; deployment code is unchanged by that removal.

The original CI failure was Chromium's setuid sandbox being unavailable inside the GitHub runner's Nix build environment. The deployed browser configuration has not been weakened to work around it.

No host activation or credential rotation performed. Isolation covers the documented backup variables; root retains access. Existing private instructions can override deployed defaults. GitHub App helper and account credentials still require private provisioning or backup restore.