fix(opencode): add Claude Opus 4.5/4.6 base variants with thinking

[johngbl]

Description

Ensures OpenCode sync exposes thinking variants for Claude Opus base models, matching the existing Claude Sonnet behavior.

Problem

build_model_catalog only defined claude-opus-4-5-thinking and claude-opus-4-6-thinking with ClaudeThinking variants. Syncing the base ids claude-opus-4-5 / claude-opus-4-6 (the ids shown by default in OpenCode) produced no variants object, so the dropdown showed without thinking options while claude-sonnet-4-6 correctly showed Low / Medium / High / Max.

Additionally, Gemini3Pro variants were inserted as low -> high -> medium which rendered in the UI as High before Low/Medium due to preserve_order JSON serialization.

Changes

• opencode_sync.rs: build_model_catalog: add claude-opus-4-5 and claude-opus-4-6 base entries with variant_type: ClaudeThinking so both base and -thinking ids expose low/medium/high/max via build_claude_thinking_variant.
• opencode_sync.rs: build_variants_object: reorder Gemini3Pro to low -> medium(disabled) -> high -> max(disabled) for consistent Default -> Low -> Medium -> High ordering.

Validation

• cargo check 0 errors
• Existing unit and integration tests unaffected (catalog order change only affects generated opencode.json)