refactor(skills): use .agents/ instead of .agent/ for phased execution

Standardize on the .agents/ directory across all phased-execution
skills (phase state, reports, sessions, validate.sh, PLAN.md, and
per-story/feature/workflow trees). Legacy dot-less agent/ fallbacks
in migration scripts are untouched.

phase-authoring/scripts/migrate-phases-to-tasks.sh

@@ -17,7 +17,7 @@
 #
 # Usage: bash migrate-phases-to-tasks.sh [project-root]
 #   project-root defaults to the nearest ancestor containing
-#   .agent/phases/todo or agent/phases/todo (older projects).
+#   .agents/phases/todo or agent/phases/todo (older projects).
 set -euo pipefail
 
 root="${1:-}"
@@ -26,14 +26,14 @@ if [[ -n "$root" ]]; then
 else
   root="$(pwd)"
   while :; do
-    if [[ -d "$root/.agent/phases/todo" || -d "$root/agent/phases/todo" ]]; then break; fi
+    if [[ -d "$root/.agents/phases/todo" || -d "$root/agent/phases/todo" ]]; then break; fi
     [[ "$root" == "/" ]] && { echo "✗ ERROR: no phases/todo found at or above $(pwd)" >&2; exit 1; }
     root="$(dirname "$root")"
   done
 fi
 
-if [[ -d "$root/.agent/phases" ]]; then
-  phases="$root/.agent/phases"
+if [[ -d "$root/.agents/phases" ]]; then
+  phases="$root/.agents/phases"
 elif [[ -d "$root/agent/phases" ]]; then
   phases="$root/agent/phases"
 else