fix(ci): stabilize and scope desktop checks

[Zongwei9888]

Summary

• synchronize the Skill creation integration test on the observable thread/start boundary
• keep the stable Desktop quality gates check while skipping heavy Desktop work for unrelated release, test, and documentation changes
• classify Desktop and bundled Python App Server inputs with unit-tested path rules
• move Desktop workflow actions off deprecated Node 20 runtimes

Verification

• uvx pre-commit run --all-files
• .venv/bin/python -m pytest -q (1265 passed, 3 skipped)
• full Desktop frontend/protocol/type/lint/test/build gate (148 tests)
• Skill creation integration test repeated 25 times
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --all-targets

[Zongwei9888]

Post-merge verification is complete. This PR was merged to main as 48c00c2. All PR checks and all checks on the merge commit passed, including Python 3.12/3.13/3.14, package validation, lint, security audit, Desktop quality gates, and the macOS arm64/x64, Windows x64, and Linux x64 bundles.

The previously flaky Skill-creation integration path passed both the PR and main Linux runners. Unrelated release/documentation/test-only changes will now keep the stable Desktop quality gates status while skipping the heavyweight Desktop build; Desktop and bundled App Server inputs still run the complete matrix.