Add WASM fallbacks for tree-sitter and SQLite, fix installer

Replace native tree-sitter with web-tree-sitter + tree-sitter-wasms for
universal cross-platform support. Add node-sqlite3-wasm as a fallback
when better-sqlite3 native bindings aren't available. Move better-sqlite3
and sqlite-vss to optionalDependencies so installs never fail.

Fix installer to use npx fallback when global npm install fails, so MCP
config, hooks, and quick-start instructions all work without the bare
codegraph command in PATH.

Fix tests: update schema version expectation, fix db test paths and
method names, extract MAX_OUTPUT_LENGTH as module constant, normalize
Windows path separators in import resolver.

scripts/postinstall.js

@@ -62,9 +62,6 @@ async function downloadModel() {
   }
 }
 
-// @sengac/tree-sitter-dart ships with NAPI prebuilds for all platforms
-// No patching needed (replaced old tree-sitter-dart v1.0.0 which used NAN bindings)
-
 downloadModel().catch(() => {
   // Silent exit - don't break npm install
   process.exit(0);