Remove MemoryDeletionRequest - migrate memory deletion to proposals-only workflow

**Current state:** We have two systems for requesting memory deletion: 1. `ask_for_memory_deletion` MCP tool → creates `MemoryDeletionRequest` → human approves → deletion 2. `create_proposal` (type: memory_cleanup) → human approves → deletion **Problem:** Two approval systems for the same purpose is redundant and confusing: - Double the code to maintain - Inconsistent user experience - Scattered audit trails **Solution:** Migrate to proposals-only workflow: 1. Deprecate and remove `ask_for_memory_deletion` MCP tool 2. Remove `MemoryDeletionRequest` model and database table 3. Update all references to use `create_proposal(type: memory_cleanup)` instead 4. Update skills/documentation to reference proposals for memory deletion **Benefits:** - Single, unified approval system - Better audit trail (all deletions tracked in proposals) - Richer context (proposals support evidence_links, reasoning, confidence) - Simpler codebase **Files affected:** - `/rails/app/models/memory_deletion_request.rb` (remove) - `/rails/app/controllers/api/v1/mcp_controller.rb` (remove ask_for_memory_deletion handler) - `/rails/config/tinker/mcp_permissions.yml` (remove permission) - Database migration to drop memory_deletion_requests table - Update any skills/docs that reference ask_for_memory_deletion