Improve ort-build skill agent tips
- Clarify build.py requires --build_dir and add direct invocation example
- Improve incremental build guidance: note when fresh build_dir is needed
- Fix --skip_tests description to reflect actual semantics
- Refine agent tips: exit code as primary success signal, specific output
types per command, precise log redirection advice, clearer long-running
build handling, and better explanation of .bat redirection issues
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
