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.

todo-to-phased/assets/phase-template.md

@@ -1,7 +1,7 @@
 # Phase {{NN}} — {{Short Title}}
 
 **Source:** `{{TODO file}} L{{range(s) the phase's tasks cover, e.g. 34–38, 41}} — "{{capability / section 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

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

@@ -1,7 +1,7 @@
 # Task {{NN}} — {{Short Title}}
 
 **Phase:** `{{NN_phase}}` · **Source:** `{{TODO file}}:{{line(s), e.g. 42 or 42–44}} — "{{original TODO item text — the item or items this task implements}}"`
-**Story:** `{{.agent/user_stories/<story>.md or "n/a"}}`
+**Story:** `{{.agents/user_stories/<story>.md or "n/a"}}`
 
 ## Objective
 {{1–2 sentences: what this task delivers}}