feat: add assistant info block to system prompt

[ferdinandyb]

Add model identity information to the system prompt, informing the assistant of its name, model ID, provider, and the current OpenCode version.

Models often misidentify themselves when asked (see #3063 where Claude Sonnet 4.5 reported itself as "Claude 3.7 Sonnet"). This adds a single sentence to the prompt to make the assistant self-aware.

Fixes #9065