Emit infer deprecation warnings during include frontmatter processing

[Copilot]

infer is marked deprecated in the workflow schema, but included-file frontmatter accepted it without any compiler signal. This change adds an explicit warning path so users see migration guidance to disable-model-invocation when infer is present.

• Deprecation signaling in parser include flow

  • Updated pkg/parser/include_processor.go to emit console.FormatWarningMessage(...) when included frontmatter contains infer.
  • Warning text points directly to the replacement field: disable-model-invocation.

• Targeted regression coverage

  • Extended pkg/parser/frontmatter_includes_test.go with a focused test that captures stderr and asserts the warning fires for infer.
  • Kept existing behavior unchanged for included files that do not use infer.

if _, hasInfer := result.Frontmatter["infer"]; hasInfer {
	fmt.Fprintln(os.Stderr, console.FormatWarningMessage(
		fmt.Sprintf("Frontmatter field 'infer' in %s is deprecated. Use 'disable-model-invocation' instead.", filePath),
	))
}