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/entrypoints/sdk/coreTypes.generated.ts

@@ -0,0 +1,2 @@
+// Stub — generated types not included in source snapshot
+export type {}

src/entrypoints/sdk/runtimeTypes.ts

@@ -0,0 +1,2 @@
+// Stub — SDK runtime types not included in source snapshot
+export type {}

src/entrypoints/sdk/toolTypes.ts

@@ -0,0 +1,2 @@
+// Stub — SDK tool types not included in source snapshot
+export type {}