fix(context): add kimi-k2.6 256k context window support (#357)

The context usage calculator was defaulting kimi-k2.6 to 200k tokens,
causing incorrect 100% context pressure alerts on new chats.

- Add 'kimi-k2.6': 256_000 to MODEL_CONTEXT_WINDOWS map
- Fixes false-positive context window warnings for kimi-k2.6 users

Refs: context-usage.ts hardcoded model list

Co-authored-by: Dev <dev@example.com>

src/server/context-usage.ts

@@ -36,6 +36,7 @@ const MODEL_CONTEXT_WINDOWS: Record<string, number> = {
   'o3-mini': 200_000,
   'gemini-2.5-flash': 1_000_000,
   'gemini-2.5-pro': 1_000_000,
+  'kimi-k2.6': 256_000,
 }
 
 const CHARS_PER_TOKEN = 3.5