feat(plugins): add Hindsight and Mem0 memory provider plugins

Adapts PR #1811 (Hindsight by benfrank241) and PR #2933 (Mem0 by
kartik-mem0) to the MemoryProvider interface as drop-in plugins.

Hindsight plugin (plugins/hindsight-memory/):
- 3 tools: hindsight_retain, hindsight_recall, hindsight_reflect
- Cloud (API key) or local (embedded PostgreSQL) modes
- Background prefetch with thread isolation for aiohttp
- Auto-sync turns to knowledge graph

Mem0 plugin (plugins/mem0-memory/):
- 4 tools: mem0_profile, mem0_search, mem0_context, mem0_conclude
- Server-side LLM fact extraction and deduplication
- Semantic search with optional reranking
- Verbatim fact storage via conclude (infer=False)

Both require API keys (HINDSIGHT_API_KEY / MEM0_API_KEY) and the
respective SDK packages (hindsight-client / mem0ai). is_available()
gates on credentials so installing the plugin without a key is safe.

plugins/hindsight-memory/plugin.yaml

@@ -0,0 +1,8 @@
+name: hindsight-memory
+version: 1.0.0
+description: >
+  Long-term memory via Hindsight — knowledge graph with entity resolution,
+  multi-strategy retrieval (semantic + BM25 + graph + temporal), and
+  cross-encoder reranking. Cloud or local mode.
+requires_env:
+  - HINDSIGHT_API_KEY