Rename directories to plural form: skill/ → skills/, agent/ → agents/, command/ → commands/

- Rename skill/ to skills/ for consistency with naming conventions
- Rename agent/ to agents/ and command/ to commands/
- Update AGENTS.md with all directory references
- Update scripts/test-skill.sh paths
- Update prompts/athena.txt documentation

This aligns with best practices of using plural directory names and updates
all documentation to reflect the new structure.

skills/plan-writing/assets/templates/project-brief.md

@@ -0,0 +1,48 @@
+# Project Brief: [Project Name]
+
+**Owner**: [Name]  
+**Timeline**: [Start Date] → [Target Date]  
+**Area**: [PARA Area, if applicable]
+
+## Goal
+
+[One clear sentence: What will be true when this project is complete?]
+
+## Success Criteria
+
+How we'll know it's done:
+
+- [ ] [Criterion 1 - specific and measurable]
+- [ ] [Criterion 2]
+- [ ] [Criterion 3]
+
+## Scope
+
+**Included**:
+- [Deliverable 1]
+- [Deliverable 2]
+
+**Not Included**:
+- [Exclusion 1]
+
+## Key Milestones
+
+| Milestone | Target Date | Status |
+|-----------|-------------|--------|
+| [Milestone 1] | [Date] | [ ] |
+| [Milestone 2] | [Date] | [ ] |
+| [Complete] | [Date] | [ ] |
+
+## Initial Tasks
+
+1. [ ] [First task to start] - Due: [Date]
+2. [ ] [Second task]
+3. [ ] [Third task]
+
+## Notes
+
+[Any context, constraints, or references worth capturing]
+
+---
+
+*Created: [Date]*