The make-shared task previously ran on every play with changed_when: false,
which always reported "ok" even when it had just mutated mount propagation.
Check /proc/self/mountinfo for the shared tag first and run mount --make-shared
only when the volume is not already shared, reporting changed accurately.

Detection is fail-safe: grep exits non-zero on no-match or any error, so the
mount is skipped only when shared propagation is positively confirmed. Every
other case falls through to running the (idempotent) command, so behavior
cannot regress relative to running it unconditionally.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>