CI: caching review + canonical path-exclusion list

[ChrisonSimtian]

Caching: actions/cache already covers ~/.nuget/packages + .fallout/temp keyed on global.json/*.csproj/Directory.Packages.props. Evaluate: include a lockfile in the key, cache build outputs, and Fallout-native incremental (future capability).

Path exclusions: define ONE canonical "never triggers CI" list (docs/, .assets/, **/*.md, likely tools/, non-workflow .github/) and apply it consistently across all workflows + the [GitHubActions] generator. Source for the path-ignore parity task.

[ChrisonSimtian]

Done in #330/#331 — canonical exclusion list applied + cache restore-keys added; caching evaluated (current dep-key adequate, no packages.lock.json, build-output caching rejected). Fallout-native incremental is future framework work, not this CI task.