fix(opencode): restore Opus 4.6 thinking sync and variant hint mapping

[d-init-d]

Summary

• fix OpenCode sync catalog mismatch by publishing claude-opus-4-6-thinking (instead of stale 4.5 id), include it in compatibility cleanup IDs, and add regression coverage for Opus 4.6 variants.
• harden Claude thinking hint extraction to read both root payload fields and providerOptions payloads, with root-level precedence and preferred provider key order.
• add roadmap/todo tracking docs for this follow-up so verification scope and known blockers are explicit.

Problem Statement

Two issues were observed after sync/variant workflows:

1. Claude Opus 4.6 Thinking could be selected in app sync UI but did not appear in OpenCode model picker.
2. Thinking variant changes (for example via Ctrl+T in OpenCode) were not reliably applied by backend mapping.

Root Cause

• OpenCode sync model catalog still registered claude-opus-4-5-thinking instead of claude-opus-4-6-thinking.
• Thinking hint extraction in Claude handler primarily relied on root-level fields and missed common providerOptions shapes.

Files Changed

• src-tauri/src/proxy/opencode_sync.rs
• src-tauri/src/proxy/handlers/claude.rs
• docs/testing/opencode_sync_followup_roadmap.md
• docs/testing/opencode_sync_followup_todolist.md

Validation

Automated

• ✅ cargo check (in src-tauri) passed.
• ❌ cargo test proxy::opencode_sync::tests --lib blocked by pre-existing unrelated compile errors in src/proxy/mappers/openai/request.rs (missing image_size/effort fields in existing tests).
• ❌ npm run build blocked by local env dependency issue: missing @tailwindcss/container-queries.

Review Gate

• ✅ kimi-k25 review: PASS.
• ✅ gemini-3-pro review: PASS.

Manual Verification Checklist

• Sync with Claude 4.6 TK selected in app.
• Confirm claude-opus-4-6-thinking exists under provider.antigravity-manager.models in OpenCode config.
• Confirm model appears in OpenCode picker.
• Switch thinking variant (e.g. Ctrl+T) and send prompt.
• Confirm proxy applies mapped thinking budget/effort from root or providerOptions hint.

Notes

• Existing baseline/environment blockers are documented in docs/testing/opencode_sync_followup_roadmap.md and docs/testing/opencode_sync_followup_todolist.md.