feat: runtime model switching via /model command

[bOsowski]

Is your feature request related to a problem?
The Claude model can only be set at startup via CLAUDE_MODEL config. Users should be able to easily switch models on the go, similar to the Claude TUI interface.

Describe the solution you'd like
A /model command that allows per-user runtime model switching:

• /model — show current model
• /model <name> — switch model (any valid Claude model name)
• /model default — reset to server/CLI default

Describe alternatives you've considered
Changing the CLAUDE_MODEL env var and restarting the bot — requires server access, causes downtime, and affects all users.

Additional context
The override is per-user (stored in context.user_data) and does not affect other users. Implementation threads model_override through the facade and SDK integration layers.