#2016 — WorktreeCreate stdout contract

Why the observer hooks look buggy but aren't — and the guard that locks it. CC reads a command-type WorktreeCreate hook's stdout as the worktree path.

Empirically verified on 8.9.1: both observer hooks emit 0 bytes stdout. Fix shipped v8.6.5 (#1996). Guard: __tests__/integration/worktree-create-stdout.test.ts.