Standardize plan and brainstorm filename conventions (#105)

- Move plans from `plans/` to `docs/plans/` for consistency with other artifacts
- Add date prefix (YYYY-MM-DD) to both plans and brainstorms for chronological sorting
- Add self-documenting suffixes: `-plan` and `-brainstorm` to identify artifact type
- Update filename derivation to extract title from content, not filename
- Clarify deepened plan naming: append `-deepened` after `-plan` suffix

New conventions:
- Brainstorms: `docs/brainstorms/YYYY-MM-DD-<topic>-brainstorm.md`
- Plans: `docs/plans/YYYY-MM-DD-<type>-<name>-plan.md`
- Deepened: `docs/plans/YYYY-MM-DD-<type>-<name>-plan-deepened.md`

plugins/compound-engineering/commands/deepen-plan.md

@@ -24,8 +24,8 @@ The result is a deeply grounded, production-ready plan with concrete implementat
 <plan_path> #$ARGUMENTS </plan_path>
 
 **If the plan path above is empty:**
-1. Check for recent plans: `ls -la plans/`
-2. Ask the user: "Which plan would you like to deepen? Please provide the path (e.g., `plans/my-feature.md`)."
+1. Check for recent plans: `ls -la docs/plans/`
+2. Ask the user: "Which plan would you like to deepen? Please provide the path (e.g., `docs/plans/2026-01-15-feat-my-feature-plan.md`)."
 
 Do not proceed until you have a valid plan file path.
 
@@ -460,7 +460,7 @@ At the top of the plan, add a summary section:
 
 ## Output Format
 
-Update the plan file in place (or create `plans/<original-name>-deepened.md` if requested).
+Update the plan file in place (or if user requests a separate file, append `-deepened` after `-plan`, e.g., `2026-01-15-feat-auth-plan-deepened.md`).
 
 ## Quality Checks