feat: add build system, stubs, and npm packaging — openclaude is now runnable

- package.json with all 70+ dependencies
- Bun build script with feature flag shims, native module stubs, otel externals
- Stubs for ~15 missing source files (snapshot gaps)
- tsconfig.json for TypeScript
- bin/openclaude entry point
- Builds to single 19MB dist/cli.mjs
- Verified: --version and --help work

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/assistant/AssistantSessionChooser.tsx

@@ -0,0 +1,10 @@
+// Stub — AssistantSessionChooser not included in source snapshot
+import React from 'react'
+
+export function AssistantSessionChooser(_props: {
+  sessions: unknown[]
+  onSelect: (id: string) => void
+  onCancel: () => void
+}) {
+  return null
+}