#13119: fix: add forward-compat for google-antigravity claude-opus-4-6 models

## Summary When antigravity updates their model catalog to include `claude-opus-4-6-thinking`, OpenClaw's pi-ai registry may not have the new ID yet, causing "Unknown model: google-antigravity/claude-opus-4-6-thinking" errors. This adds a forward-compat fallback that maps `claude-opus-4-6` variants to their `claude-opus-4-5` equivalents for the `google-antigravity` provider, similar to the existing `anthropic` provider handling in `resolveAnthropicOpus46ForwardCompatModel`. ## Changes - Added `resolveAntigravityOpus46ForwardCompatModel()` function that: - Detects claude-opus-4-6 model IDs for google-antigravity provider - Maps them to claude-opus-4-5 equivalents in the registry - Falls back to synthetic model definition if no template found - Properly sets `reasoning: true` for `-thinking` variants ## Models Fixed - `google-antigravity/claude-opus-4-6` - `google-antigravity/claude-opus-4-6-thinking` - `google-antigravity/claude-opus-4.6` ## Test Plan - [ ] Verify `google-antigravity/claude-opus-4-6-thinking` resolves without "Unknown model" error - [ ] Verify model inherits correct settings from claude-opus-4-5-thinking template - [ ] Verify `-thinking` variants have `reasoning: true` 🤖 Generated with [Claude Code](https://claude.ai/code) <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This change adds a `google-antigravity`-specific forward-compat resolver in `src/agents/pi-embedded-runner/model.ts` that detects `claude-opus-4-6` / `claude-opus-4.6` (and `-thinking` variants) and maps them to `claude-opus-4-5` / `claude-opus-4.5` template models from the pi-ai registry when available. If no template exists, it synthesizes a minimal `anthropic-messages` model definition, setting `reasoning: true` when the requested model id contains `thinking`. The resolver is wired into `resolveModel()` alongside the existing OpenAI Codex and Anthropic Opus forward-compat fallbacks, so users won’t hit `Unknown model: google-antigravity/...` when antigravity’s catalog updates faster than the local registry. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is narrowly scoped to model resolution fallback logic, follows the existing forward-compat pattern used for other providers, and only activates for the `google-antigravity` provider with specific `claude-opus-4-6` model id patterns. No functional regressions were identified in the surrounding call sites. - src/agents/pi-embedded-runner/model.ts <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->