Add a SHA-256 content-hash based build stamp to `hermes desktop` so
unchanged source trees skip the npm install + build step. Uses pathspec
for .gitignore-aware file matching instead of a hardcoded skip-list.
New CLI flags:
- --build-only: run the build but don't launch the app
- --force-build: rebuild even when the stamp matches
`hermes update` now calls `hermes desktop --build-only` so the
desktop app is rebuilt (if needed) as part of the update flow.
16/16 tests passing.
c2050183a5