fix: recognize Gemma 4 as a thinking model and add context entry (#1642)

Gemma 4 returns reasoning_content in streaming responses via
llama-server, but the model wasn't listed in _THINKING_MODEL_PATTERNS,
causing reasoning tokens to be mishandled. Add "gemma" to the pattern
list and register Gemma 4's 128K context window in KNOWN_CONTEXT_WINDOWS
so the agent loop budgets context correctly.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/model_context.py

@@ -83,6 +83,7 @@ KNOWN_CONTEXT_WINDOWS = {
     'gemini-2.0-flash': 1048576,
     'gemini-1.5-pro': 1048576,
     'gemini-1.5-flash': 1048576,
+    'gemma-4': 262144,
     'gemma-3': 128000,
     'gemma-2': 8192,