feat(plugins): add OpenViking, RetainDB, and Cognitive memory providers

Adapts three more memory backend PRs to the MemoryProvider interface:

OpenViking (PR #3369 by Mibayy):
- 3 tools: viking_search, viking_read, viking_browse
- Read-only, self-hosted server, no sync/prefetch
- URI-based content with progressive disclosure levels

RetainDB (PR #2732 by Alinxus):
- 5 tools: retaindb_profile, retaindb_search, retaindb_context,
  retaindb_remember, retaindb_forget
- Cloud API with prefetch, sync, and memory bridging
- Durable write-behind queue pattern

Cognitive Memory (PR #727 by 0xbyt4):
- 1 tool with 4 actions: recall, store, forget, status
- Local SQLite with vector embeddings (litellm)
- Auto-classification, importance decay, dedup, forgetting

All gated on credentials/deps via is_available():
- OpenViking: OPENVIKING_ENDPOINT + server health check
- RetainDB: RETAINDB_API_KEY
- Cognitive: litellm importable (uses its env vars for embedding API)

plugins/openviking-memory/plugin.yaml

@@ -0,0 +1,7 @@
+name: openviking-memory
+version: 1.0.0
+description: >
+  Read-only memory via OpenViking — semantic search, URI-based content
+  reading, and filesystem browsing over a self-hosted knowledge server.
+requires_env:
+  - OPENVIKING_ENDPOINT