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/phase-template.md

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

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

@@ -31,4 +31,4 @@ accessibility, asset policy) — whichever matches the project's domain.}}
 | # | Phase | Feature file | Objective (one line) |
 |---|-------|--------------|----------------------|
 | 01 | {{NN_name}} | — | foundation: adapt validate.sh, green test baseline |
-| 02 | {{NN_name}} | `.agent/{{workflows\|features\|user_stories}}/{{name}}.md` | {{...}} |
+| 02 | {{NN_name}} | `.agents/{{workflows\|features\|user_stories}}/{{name}}.md` | {{...}} |

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)}}