feat: expose acp thinking variants

[validatedev]

What does this PR do?

Fixes #8973 by exposing thinking variants in ACP model selection:

• Appends model+variant entries (e.g., claude-sonnet-4/high) to models.availableModels
• Parses variant from session/set_model selections and keeps session variant in sync
• Returns proper variant metadata via _meta (modelId, variant, availableVariants) instead of empty object
• Updated @agentclientprotocol/sdk to 0.13.0

How did you verify your code works?

• bun run typecheck
• Zed exposes thinking variants via model selector
• Verified session/set_model responses include correct _meta.opencode with variant info

Screenshot