fix(agent-branch-start): verify worktree exists before printing Ready: (#577)

`git worktree add` has been observed to return exit code 0 with the
target directory missing on disk. The script then ran OpenSpec /
dependency-symlink init against the phantom path, eventually printing
"[agent-branch-start] Ready:" and returning success. Callers cd into
the vanished directory, claim file locks against it, and silently
lose every subsequent edit until the next finish step complains
about a missing tree.

Two defensive checks added:

1. Right after `git worktree add` returns 0 — fail fast before the
   init phase touches anything in the phantom path.
2. At the top of `print_agent_next_steps` — covers the case where
   the dir gets cleaned up by something during init (less likely,
   but cheap to verify).

Both report the branch name and expected path, and tell the operator
to run `git worktree prune` before retry.

Co-authored-by: NagyVikt <nagy.viktordp@gmail.com>

Author: NagyVikt

templates/scripts/agent-branch-start.sh

@@ -417,6 +417,20 @@ print_agent_next_steps() {
     base_branch="main"
   fi
 
+  # Pre-`Ready:` post-condition check. `git worktree add` has been observed
+  # to return 0 with the worktree dir still missing on disk (race condition
+  # during the OpenSpec / dependency-symlink phase between `worktree add`
+  # and now). If we print `Ready:` in that state, callers cd into a
+  # vanished directory and lose subsequent edits silently. Surface it as
+  # an exit-1 error instead — operator can retry + `git worktree prune`.
+  if [[ ! -d "$worktree_path" || ! -e "$worktree_path/.git" ]]; then
+    printf '[agent-branch-start] ERROR: worktree did not materialize on disk before Ready:\n' >&2
+    printf '[agent-branch-start]   branch:   %s\n' "$branch_name" >&2
+    printf '[agent-branch-start]   expected: %s\n' "$worktree_path" >&2
+    printf '[agent-branch-start] Run `git worktree prune` to clear ghost entries, then retry.\n' >&2
+    exit 1
+  fi
+
   echo "[agent-branch-start] Ready:"
   echo "  branch:   ${branch_name}"
   echo "  worktree: ${worktree_path}"
@@ -864,6 +878,13 @@ if ! worktree_add_output="$(git -C "$repo_root" worktree add -b "$branch_name" "
   printf '%s\n' "$worktree_add_output" >&2
   exit 1
 fi
+# git worktree add has been observed to exit 0 while the target dir is
+# missing — verify before downstream init runs against a phantom path.
+if [[ ! -d "$worktree_path" || ! -e "$worktree_path/.git" ]]; then
+  printf '[agent-branch-start] ERROR: git worktree add reported success but %s is not a valid worktree.\n' "$worktree_path" >&2
+  printf '%s\n' "$worktree_add_output" >&2
+  exit 1
+fi
 git -C "$repo_root" config "branch.${branch_name}.guardexBase" "$BASE_BRANCH" >/dev/null 2>&1 || true
 git -C "$repo_root" config "branch.${branch_name}.guardexWorktreeRoot" "$WORKTREE_ROOT_REL" >/dev/null 2>&1 || true
 # Fresh agent branches should start unpublished; clear any inherited base-branch tracking.