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/skills/brainstorming/SKILL.md

@@ -134,7 +134,7 @@ topic: <kebab-case-topic>
 → `/workflows:plan` for implementation details
 ```
 
-**Output Location:** `docs/brainstorms/YYYY-MM-DD-<topic>.md`
+**Output Location:** `docs/brainstorms/YYYY-MM-DD-<topic>-brainstorm.md`
 
 ### Phase 4: Handoff