Play Framework

fix: playDeps defaults to sync ON, opt-out via -Pnosync (PF-90)

Got the python semantics backwards in the previous commit. Re-reading
DependenciesManager.sync() and deps.py:
- play deps        → resolve + install + AUTO-SYNC (delete stale)
- play deps --sync → identical (--sync is redundant explicit-opt-in)
- play deps --nosync → resolve + install, NO sync (stale preserved)

Only --nosync changes behavior. So the Gradle equivalent should default
sync ON and offer -Pnosync to opt out. Previous commit had it inverted
(opt-in via -Psync), which would silently break Dockerfiles inheriting
the auto-sync expectation from the python era.

Verified: plain `gradle playDeps` removes stale jars; `-Pnosync`
preserves them.