fix(dev): agent.sh worktree_count/paths grep exit-1 on empty set

grep -cv (and grep -v) return exit code 1 when there are zero matches.
With set -euo pipefail this silently aborted the script before count
was returned — causing 'agent.sh new' to fail on a fresh repo with no
existing worktrees.

Fix: move the grep -v into worktree_paths with '|| true' so the
function always exits 0, then derive worktree_count via wc -l.

scripts/dev/agent.sh

@@ -36,22 +36,18 @@ require_clean_tree() {
   [ -z "$dirty" ] || prefail "working tree is not clean — stash or commit first"
 }
 
-worktree_count() {
-  # count registered worktrees that are NOT the main checkout
+worktree_paths() {
+  # list worktree paths (excluding main); || true prevents grep exit-1 when empty
   local main_path
   main_path=$(git rev-parse --show-toplevel)
   git worktree list --porcelain \
     | awk '/^worktree /{p=$2} /^$/{print p}' \
-    | grep -cv "^${main_path}$"
+    | grep -v "^${main_path}$" \
+    || true
 }
 
-worktree_paths() {
-  # list worktree paths (excluding main)
-  local main_path
-  main_path=$(git rev-parse --show-toplevel)
-  git worktree list --porcelain \
-    | awk '/^worktree /{p=$2} /^$/{print p}' \
-    | grep -v "^${main_path}$"
+worktree_count() {
+  worktree_paths | wc -l
 }
 
 branch_exists_local()  { git show-ref --verify --quiet "refs/heads/$1"; }