feat(memory): add delete fact and clear all memory endpoints

[mvanhorn]

Summary

Adds memory management capabilities. The backend API now supports deleting individual facts and clearing all memory. The frontend memory settings page gets interactive delete buttons per fact and a "Clear All Memory" button with confirmation dialog.

Why this matters

Issue #1333 requests memory clearing/management. The current API only supports reading memory data (GET /api/memory) and reloading from file (POST /api/memory/reload). There is no way to delete facts or reset memory. The settings page renders memory as read-only markdown.

Changes

Backend (harness layer - deerflow.*)

• delete_memory_fact(fact_id) in updater.py - removes a single fact by ID, returns False if not found
• clear_memory() in updater.py - resets memory to empty state via _create_empty_memory() + _save_memory_to_file()

Backend (app layer - app.*)

• DELETE /api/memory/facts/{fact_id} - returns 200 on success, 404 if fact not found
• DELETE /api/memory - clears all memory, returns the empty state

Frontend

• deleteMemoryFact() and clearAllMemory() API functions in core/memory/api.ts
• useDeleteFact() and useClearMemory() TanStack Query mutation hooks with cache invalidation
• Interactive FactsTable component replaces the markdown table - each row has a trash icon button
• "Clear All Memory" button with confirmation dialog at the top of the memory section

Testing

6 unit tests covering:

• Delete existing fact (verifies removal + other facts preserved)
• Delete nonexistent fact (returns False, no side effects)
• Delete from empty facts list
• Clear resets to empty state (all sections zeroed)
• Clear updates lastUpdated timestamp
• Clear when already empty (idempotent)

Harness boundary test passes - no app.* imports in deerflow.*.

Video Demo

Fixes #1333

This contribution was developed with AI assistance (Claude Code).