fix: plugin no longer vanishes after Claude Code updates

Root cause: plugin registered as directory source in known_marketplaces.json,
which gets wiped on CLI updates. Now registers in extraKnownMarketplaces
(settings.json) as a GitHub source — same mechanism caveman/context-mode use.

Binaries install to ~/.claude-mnemonic/bin/ instead of the Claude-managed
plugins directory. Thin wrapper scripts in the repo let the marketplace
clone find them. Nothing gets cleaned up when Claude refreshes its cache.

Also fixed along the way:
- ONNX Runtime 1.24.3 → 1.26.0 (API v25 mismatch broke all embedding tests)
- Vector client leaked on DB reinit, processQueue had a race on sessionManager
- reloadConfig called os.Exit(0) bypassing graceful shutdown
- Removed dead QueryRowWithTimeout that leaked contexts
- Added tests for graph/watcher/maintenance/update (all were at 0%)

.claude-plugin/plugin.json

@@ -0,0 +1,22 @@
+{
+  "name": "claude-mnemonic",
+  "version": "v0.11.57-dirty",
+  "description": "Persistent memory system for Claude Code with SQLite, FTS5, and vector search",
+  "author": {
+    "name": "lukaszraczylo",
+    "email": "lukaszraczylo@users.noreply.github.com",
+    "url": "https://github.com/lukaszraczylo"
+  },
+  "homepage": "https://github.com/lukaszraczylo/claude-mnemonic",
+  "repository": "https://github.com/lukaszraczylo/claude-mnemonic",
+  "license": "MIT",
+  "hooks": "./hooks/hooks.json",
+  "mcpServers": {
+    "claude-mnemonic": {
+      "command": "${CLAUDE_PLUGIN_ROOT}/mcp-server",
+      "args": ["--project", "${CLAUDE_PROJECT}"],
+      "env": {}
+    }
+  },
+  "commands": ["./commands/restart.md"]
+}