fix(npm): download binaries during postinstall

The npm package was missing the actual binary download step - users got
wrapper scripts pointing to non-existent binaries.

Changes:
- postinstall.js now downloads correct binary from GitHub releases
- Added vestige.js wrapper for CLI binary
- Exposed both vestige-mcp and vestige commands in package.json
- Updated README with troubleshooting, storage info, CLI docs
- Added .gitignore for downloaded binaries

Fixes fresh install issues where Claude Desktop couldn't attach and
vestige CLI wasn't found.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/vestige-mcp-npm/.gitignore

@@ -0,0 +1,7 @@
+# Downloaded binaries (installed via postinstall)
+bin/vestige
+bin/vestige.exe
+bin/vestige-mcp
+bin/vestige-mcp.exe
+bin/*.tar.gz
+bin/*.zip