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.

convert-to-phased/assets/task-template.md

@@ -1,6 +1,6 @@
 # Task {{NN}} — {{Short Title}}
 
-**Phase:** `{{NN_phase}}` · **Feature:** `{{.agent/workflows/<name>.md | .agent/features/<name>.md | .agent/user_stories/<name>.md | "n/a"}}`
+**Phase:** `{{NN_phase}}` · **Feature:** `{{.agents/workflows/<name>.md | .agents/features/<name>.md | .agents/user_stories/<name>.md | "n/a"}}`
 
 ## Objective
 {{1–2 sentences: what this task delivers (the delta from current state to the contract)}}