chore: update COPILOT_CLI_VERSION to 1.0.43 (#436)

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

src/engine.rs

@@ -42,7 +42,7 @@ pub const DEFAULT_COPILOT_MODEL: &str = "claude-opus-4.7";
 
 /// Default pinned version of the Copilot CLI NuGet package.
 /// Override per-agent via `engine: { id: copilot, version: "1.0.35" }` in front matter.
-pub const COPILOT_CLI_VERSION: &str = "1.0.40";
+pub const COPILOT_CLI_VERSION: &str = "1.0.43";
 
 /// Resolved engine — enum dispatch over supported engine identifiers.
 ///