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/assets/phase-template.md

@@ -1,6 +1,6 @@
 # Phase {{NN}} — {{Short Title}}
 
-**Story:** `{{.agent/user_stories/<story>.md or "n/a"}}`
+**Story:** `{{.agents/user_stories/<story>.md or "n/a"}}`
 **Context:** `{{PLAN.md sections / files this phase builds on}}`
 
 ## Objective