fix(ripgrep): use @vscode/ripgrep package as the builtin source (#911) (#932)

The vendored-binary lookup at vendor/ripgrep/<arch>-<platform>/rg never
resolved in this fork — that directory does not ship — so users without
a system rg had no working fallback. Switch to the @vscode/ripgrep
package so Microsoft maintains the platform/arch matrix and the binary
is delivered via npm.

- src/utils/ripgrep.ts: replace hand-rolled vendor-path resolution with
  rgPath from @vscode/ripgrep. Lazy require so a missing package falls
  through to the system rg branch instead of throwing at import.
  Drop builtinExists from the config args; builtinCommand is now a
  string-or-null. The system override (USE_BUILTIN_RIPGREP=0), the
  Bun-compiled standalone embedded mode, the macOS codesign hook, and
  all retry/timeout/error logic are preserved untouched.
- scripts/build.ts: mark @vscode/ripgrep as external. The package
  resolves rgPath via __dirname at runtime, so bundling would freeze
  the build host's absolute path into dist/cli.mjs.
- src/utils/ripgrep.test.ts: update for the new config shape and add
  tests covering USE_BUILTIN_RIPGREP=0, embedded mode, last-resort
  fallback, and null builtin path.

Tested locally on Linux (Bun 1.3.13). macOS (codesign hook) and
Windows (rg.exe extension) need contributor verification.

scripts/build.ts

@@ -472,6 +472,11 @@ ${exports}
     '@aws-sdk/credential-providers',
     '@azure/identity',
     'google-auth-library',
+    // @vscode/ripgrep ships a platform-specific binary alongside its
+    // index.js and resolves the path via __dirname at runtime. Bundling
+    // would freeze the build host's absolute path into dist/cli.mjs, so we
+    // keep it external and rely on the npm package being installed.
+    '@vscode/ripgrep',
   ],
 })