docs(zen): update GPT 5.6 Sol pricing

[opencode-agent]

Summary

• update GPT 5.6 Sol's discounted Zen pricing for short and long contexts
• update cached read and cached write prices alongside input and output prices
• retain Zen's September 18, 2026 end date for the 50% discount
• apply the changes across all 18 localized Zen pricing pages

Pricing

• ≤272K tokens: $2 input, $10 output, $0.20 cached read, $2.50 cached write per 1M tokens

• 272K tokens: $4 input, $15 output, $0.40 cached read, $5 cached write per 1M tokens

Source pricing: https://developers.openai.com/api/docs/pricing

Checks

• git diff --check
• verified all 18 localized pages contain the updated short-context prices, long-context prices, and Zen promotional end date

Requested by: @Slickstef11 (Stefan via Slack)