Log the shared-worktree collision in THREADS

Lane D is working in the shared ~/Documents/shades checkout rather than its own
clone, so its git checkout moved HEAD off lane/a mid-session and Lane A's M0
commits landed straight on main. No damage this time, but two agents sharing one
HEAD and one index will eventually eat someone's uncommitted work.

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

THREADS.md

@@ -70,6 +70,17 @@ Format: `[lane letter] YYYY-MM-DD — note`
     character is still reachable; Running / Falling / Crouch-To-Stand / crank / dig are not sourced.
     Don't spend a day looking — answer is needed from whoever wrote §2.
 
+[A] 2026-07-16 — ⚠️ **WORKTREE COLLISION — happened today, please fix.** Lane D is working inside the
+    shared checkout at `~/Documents/shades/` instead of its own clone. Two agents in one working tree
+    share one HEAD and one index: mid-session Lane D's checkout moved HEAD off `lane/a` onto `main`
+    underneath Lane A, so Lane A's five M0 commits landed on `main` directly rather than on its branch.
+    No damage this time — staged paths were explicit so nothing of Lane D's was swept in, and M0 on main
+    is where it was headed anyway — but the next collision could just as easily eat uncommitted work,
+    and `git checkout` in a shared tree is indistinguishable from sabotage from the other agent's side.
+    Lane A has moved out to `~/Documents/shades-laneA/`. **Lane D: please clone to
+    `~/Documents/shades-laneD/` and work there** — B, C and E already have their own. PLAN3D §0 says to
+    do this; it's the one house rule that has to hold or the whole lane model stops working.
+
 [A] 2026-07-16 — ❓ **OPEN QUESTION, needs a human.** PLAN3D §0 says lanes run on "the M1 Ultra
     (`johnking@100.91.239.7`, Tailscale)", but this box is `m3ultra` and already has
     `~/Documents/shades-laneB/` and `shades-laneE/` checked out — so lanes are in fact running here, and