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.

phased-execution/scripts/run-phase.sh

@@ -2,7 +2,7 @@
 # run-phase.sh — run every remaining task of one phase to completion.
 #
 # Each task runs in its own pi process (fresh context) with the
-# .agent/validate.sh gate after every task; the phase ends with its
+# .agents/validate.sh gate after every task; the phase ends with its
 # 00_phase.md final pass.
 #
 # Usage:
@@ -16,12 +16,12 @@ set -uo pipefail
 SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "$SCRIPT_DIR/lib.sh"
 
-# Make interruptions visible: state stays in .agent/phases/todo, and the
+# Make interruptions visible: state stays in .agents/phases/todo, and the
 # failed executor's session is still resumable on the next run.
 trap 'echo; echo "✗ ERROR: interrupted (SIGINT) — ${phase:-this phase} is left in $PHASE_TODO/; re-run to continue" >&2; exit 130' INT
 trap 'echo; echo "✗ ERROR: interrupted (SIGTERM) — ${phase:-this phase} is left in $PHASE_TODO/; re-run to continue" >&2; exit 143' TERM
 
-cd "$(find_root)" || die "no .agent/phases/todo found in this or parent directories (run /to-phase or /audit-create first)"
+cd "$(find_root)" || die "no .agents/phases/todo found in this or parent directories (run /to-phase or /audit-create first)"
 
 phase="${1:-}"
 if [[ -n "$phase" ]]; then