The ACP fs bridge's worktree guard used an unbounded `startsWith(resolve(
worktreePath))`, so a sibling path sharing the worktree as a string prefix
(`<worktree>-evil/...`) escaped the scope. Since writeWorktreeTextFile hits disk
directly (no pending_changes gate), a confused/buggy ACP agent could write
outside its worktree. Now uses a separator-bounded check matching write_guard.ts
(resolve() + `startsWith(root + sep)` / `=== root`) via a shared resolveInWorktree,
with a regression test (../ traversal + the sibling-prefix bug). Symlink-swap
hardening intentionally skipped — consistent with write_guard's no-realpath
stance; the agent runs with host FS access so this is a containment guard, not a
trust boundary. Flagged by the automated push security review.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
