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%)

internal/embedding/assets/lib/darwin-arm64/.version

@@ -1 +1 @@
-1.24.3
+1.26.0

internal/embedding/assets/lib/linux-amd64/.version

@@ -1 +1 @@
-1.24.3
+1.26.0

internal/embedding/assets/lib/linux-arm64/.version

@@ -1 +1 @@
-1.24.3
+1.26.0

internal/embedding/assets/lib/windows-amd64/.version

@@ -1 +1 @@
-1.24.3
+1.26.0