feat(agents): replace Gemini default with Antigravity

[cbusillo]

Summary

• replace built-in Gemini CLI defaults with the Antigravity (agy) agent selector
• preserve Gemini chat/review model config while migrating legacy Gemini agent selectors to Antigravity
• pass repo/worktree paths to agy via --add-dir so Antigravity can inspect the active workspace
• update docs, prompts, schema text, install guidance, and tests for agent/model selector terminology

Validation

• cargo test -p code-core --test antigravity_agent_spec
• cargo test -p code-core upgrade_legacy_model_slugs
• cargo test -p code-core resolve_agent_command_for_check_tests::antigravity_fallback_uses_agy_for_command_checks
• cargo test -p code-core antigravity_receives_workspace_add_dir
• cargo test -p code-tui build_turn_message_includes_agent_guidance
• git diff --check
• ./build-fast.sh
• just local-code-rebuild
• uv run /Users/cbusillo/Developer/codex-skills/jetbrains-inspection/scripts/jb-inspect.py run --repo "$PWD" --scope changed_files
• Dogfood: fake /v1/responses harness smoke passed
• Dogfood: live read-only antigravity agent smoke returned ANTIGRAVITY_RESTART_OK and REPO_VISIBLE=yes