Second Brain UI: Added project and agent filters

## What
Added filter dropdowns to all memory listing pages in Second Brain web UI.

## Pages Updated
- /memories (main listing)
- /specs
- /conversations  
- /decisions
- /research
- /tasks
- /context (NEW page)

## Filters Added
1. **Project filter**: MedSchools.ai, WiderWings, Second Brain, Operations
2. **Agent filter**: Shows name + role (e.g. "Bob (Chief of Staff)")

## Technical Details
- Added brain_agents to Supabase types
- Each page.server.ts loads agents list
- Filters by created_by and project_id fields
- Agent dropdown shows description field for role

## Commits
- 8d16e4a: Added project+agent filters
- 50fe088: Added Context page + agent roles in dropdowns