docs(cross-project): prefer inbox-send for handoff drops

The cross-project boundary rule already prescribes dropping a handoff
file in the target project's inbox/ when option 1 ("do it from here")
is chosen. The change adds a paragraph pointing at the new inbox-send
script as the preferred tool for the drop. The script handles project
discovery, source-project provenance in the filename, slug derivation,
and timestamping in one call, replacing the hand-constructed filenames
and guessed project paths the rule used to require.

Filename convention is unchanged
(YYYY-MM-DD-HHMM-from-<source>-<slug>.<ext>); the script just generates
it instead of me typing it out. Fallback to Write/Edit is documented
for cases where the script isn't installed (e.g. a freshly-cloned
project before the first startup-rsync).

0 files changed, 0 insertions, 0 deletions