feat: add Smart Fridge shell and per-credential shell routing

Implement Samsung FridgeOS-themed shell (PLAN.md §3.3) with inventory
management, temperature controls, diagnostics, alerts, and other
appliance commands. Add per-credential shell routing so static
credentials can specify which shell to use via the `shell` config field,
passed through ssh.Permissions.Extensions.

Also extract shared ReadLine helper from bash to the shell package so
both shells can reuse terminal input handling.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -33,7 +33,8 @@ Key settings:
 - `ssh.host_key_path` — Ed25519 host key, auto-generated if missing
 - `auth.accept_after` — accept login after N failures per IP (default `10`)
 - `auth.credential_ttl` — how long to remember accepted credentials (default `24h`)
-- `auth.static_credentials` — always-accepted username/password pairs
+- `auth.static_credentials` — always-accepted username/password pairs (optional `shell` field routes to a specific shell)
+- Available shells: `bash` (fake Linux shell), `fridge` (Samsung Smart Fridge OS)
 - `storage.db_path` — SQLite database path (default `oubliette.db`)
 - `storage.retention_days` — auto-prune records older than N days (default `90`)
 - `storage.retention_interval` — how often to run retention (default `1h`)