chore: update Claude 4.6 context window to 1M (GA)

Claude Opus 4.6 and Sonnet 4.6 1M context window is now GA and no
longer requires a beta header. Update contextWindow from 200k to 1M
for all OpenClaw/OpenCode presets (27 entries in OpenClaw, 1 in
OpenCode Bedrock). Also add claude-sonnet-4-6 model pricing seed.

src/config/opencodeProviderPresets.ts

@@ -170,7 +170,7 @@ export const OPENCODE_PRESET_MODEL_VARIANTS: Record<
     {
       id: "global.anthropic.claude-sonnet-4-6",
       name: "Claude Sonnet 4.6",
-      contextLimit: 200000,
+      contextLimit: 1000000,
       outputLimit: 64000,
       modalities: { input: ["text", "image", "pdf"], output: ["text"] },
     },