feat(opencode): enable 1M context + adaptive thinking for Opus 4.6 (#150)

[randomm]

Summary

Minimal port of upstream anomalyco#12338 to enable 1M context window and adaptive thinking for Claude Opus 4.6.

Changes

• Added context-1m-2025-08-07 and adaptive-thinking-2026-01-28 beta headers to Anthropic provider
• Added Opus 4.6 model detection (claude-opus-4.6 / claude-opus-4-6)
• Opus 4.6 uses effort-based adaptive thinking (high/max) instead of fixed budgetTokens
• No dependency changes required

Testing

• TypeScript typecheck passes
• All 1308 tests pass
• Manual testing needed: Opus 4.6 with >200k tokens

Upstream Reference

• Issue: 1M tokens for Opus 4.6 anomalyco/opencode#12338
• PR: feat(provider): add adaptive thinking and 1M context support for Claude Opus 4.6 anomalyco/opencode#12342

Fixes #150

[github-actions]

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