fix(/status): use CLAUDE_MODELS.DEFAULT instead of stale 'claude-sonnet-4.5' fallback (#723)

The /status slash command's response object falls back to a hardcoded
'claude-sonnet-4.5' when no session model is set in context. Two issues:

1. 'claude-sonnet-4.5' is not in CLAUDE_MODELS.OPTIONS (the dropdown), so
   the displayed model can be a value the user can't actually select.
2. CLAUDE_MODELS.DEFAULT is currently "opus" — meaning a fresh session
   started with no model picked actually runs on opus, not sonnet-4.5.
   The /status command therefore mis-reports the running model.

Tracking the centralized DEFAULT keeps the displayed value coherent with
what claude-sdk.js actually spawns (line 208 already uses
`options.model || CLAUDE_MODELS.DEFAULT`).

Co-authored-by: Enrico Masella <enrico.masella@gmail.com>
Co-authored-by: Simos Mikelatos <simosmik@gmail.com>

server/routes/commands.js

@@ -320,7 +320,7 @@ Custom commands can be created in:
         packageName,
         uptime: uptimeFormatted,
         uptimeSeconds: Math.floor(uptime),
-        model: context?.model || 'claude-sonnet-4.5',
+        model: context?.model || CLAUDE_MODELS.DEFAULT,
         provider: context?.provider || 'claude',
         nodeVersion: process.version,
         platform: process.platform