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).

1 files changed, 10 insertions, 0 deletions

diff --git a/claude-rules/cross-project.md b/claude-rules/cross-project.md
index b451c5a..50bc34e 100644
--- a/claude-rules/cross-project.md
+++ b/claude-rules/cross-project.md
@@ -25,6 +25,16 @@ State the mismatch and offer the two acceptable answers. Inline numbered options
 Two acceptable outcomes:
 
 1. **"Yes, do it from here"** — proceed. Record the cross-project artifact in a handoff file under the *other* project's `inbox/`, named `YYYY-MM-DD-handoff-from-<this-project>-<topic>.org`, with a top note explaining the crossover. The other project's startup workflow picks it up during inbox processing.
+
+   Prefer the `inbox-send` script (`.ai/scripts/inbox-send.py`, auto-synced into every project) over a manual `Write`/`Edit` for the drop. It handles project discovery, source-project provenance in the filename, slug derivation, and timestamping in one call:
+
+   ```
+   inbox-send <target> --text "your message"          # text → dated .org file
+   inbox-send <target> --file <path>                  # copy a file into target/inbox/
+   inbox-send --list                                  # see available projects
+   ```
+
+   Output filenames follow `YYYY-MM-DD-HHMM-from-<this-project>-<slug>.<ext>` automatically, so the target's next session sees the source + timestamp at a glance without you having to construct the name. Fall back to `Write`/`Edit` only when the script isn't available (e.g. a freshly-cloned project before the first startup-rsync).
 2. **"Switch projects"** — stop. Let the user reopen Claude in the right cwd.
 
 Don't assume which one was meant. Either guess is wrong half the time and the cost of asking once is one short turn.