an0 reject scalar SYNC_DESTINATIONS instead of word-splitting it

A scalar SYNC_DESTINATIONS containing spaces was word-split by the shared
resolve_config_array helper, breaking a single destination into multiple
broken arguments passed to rsync.

A list of rsync destinations is inherently a list, and array syntax is the
only spelling that preserves embedded spaces. resolve_sync_destinations now
detects a scalar declaration via declare -p and fails with a clear
config_error telling the user to use array syntax. The array path is
unchanged, and resolve_config_array's scalar word-splitting is left intact
for TAR_OPTS (where turning "-c -v" into separate options is desired).

Adds focused tests proving the scalar case errors without invoking rsync and
the array case preserves a space-containing destination as one argument.
Updates src/shuriken.default.conf and README.md to document the requirement.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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