fix: use platform-aware local defaults

- Route app DB and sqlite-vec defaults through shared platform helpers
- Mirror platform DB defaults in the standalone MCP package
- Fix Windows setup npm spawning and remove macOS-only env defaults

.env.example

@@ -6,9 +6,10 @@
 # Get one at: https://platform.openai.com/api-keys
 OPENAI_API_KEY=
 
-# Database path (defaults work for most users)
-# SQLITE_DB_PATH=~/Library/Application Support/RA-H/db/rah.sqlite
-SQLITE_VEC_EXTENSION_PATH=./vendor/sqlite-extensions/vec0.dylib
+# Database/vector paths are auto-detected for macOS, Windows, and Linux.
+# Override only if you intentionally want a custom location.
+# SQLITE_DB_PATH=/absolute/path/to/rah.sqlite
+# SQLITE_VEC_EXTENSION_PATH=/absolute/path/to/vec0.<dylib|dll|so>
 
 # App config (no changes needed)
 NODE_ENV=development