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%)

.gitignore

@@ -82,11 +82,7 @@ logs/
 # goreleaser
 dist/
 docs/dist
-.claude-plugin
 
-# Auto-generated plugin configs (generated by scripts/generate-plugin-config.sh)
-.claude-plugin/
-
-# Non-template plugin configs (keep only .tpl files)
+# Non-template plugin configs (keep only .tpl files in plugin/ dir)
 plugin/.claude-plugin/plugin.json
 plugin/.claude-plugin/marketplace.json