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/phase-status.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/env bash
 # phase-status.sh — phase/task pipeline state for the Phase Architect.
 #
-# Finds the project root (nearest ancestor with .agent/phases/todo), prints
+# Finds the project root (nearest ancestor with .agents/phases/todo), prints
 # the todo/ and complete/ phase listings with per-task state, the next free
 # phase number NN (counting both directories together, zero-padded to 2
 # digits), and the next pending unit in pipeline order.
@@ -15,12 +15,12 @@ set -euo pipefail
 
 root="$(pwd)"
 while :; do
-  if [[ -d "$root/.agent/phases/todo" ]]; then break; fi
-  [[ "$root" == "/" ]] && { echo "✗ ERROR: no .agent/phases/todo found at or above $(pwd)" >&2; exit 1; }
+  if [[ -d "$root/.agents/phases/todo" ]]; then break; fi
+  [[ "$root" == "/" ]] && { echo "✗ ERROR: no .agents/phases/todo found at or above $(pwd)" >&2; exit 1; }
   root="$(dirname "$root")"
 done
 
-phases="$root/.agent/phases"
+phases="$root/.agents/phases"
 
 # --- todo: per-task state ------------------------------------------------------
 echo "todo:"