Add Antigravity trace doctor support

[Stephen Belanger (Qard)]

Summary

• expose Antigravity through bt trace doctor
• map it to the existing antigravity tracing source and Google Antigravity display name
• extend the supported-agent doctor test to cover parsing, source identity, and display names

This does not change the Antigravity plugin payload or its install/uninstall behavior.

Validation

• cargo fmt --manifest-path bt-daemon/Cargo.toml -- --check
• cargo test --manifest-path bt-daemon/Cargo.toml --all-features --locked
• cargo clippy --manifest-path bt-daemon/Cargo.toml --all-targets --all-features --locked -- -D warnings
• make validate-antigravity
• git diff --check

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	⚠️ Failed 2026-09-01T11:07:48.059853Z	5ffd4a5	PR opened

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