build: migrate from pnpm to bun as package manager (#28)

## Summary
- Replace pnpm with bun for dependency installation, script running, and ad-hoc execution
- Keep pnpm for publish workflow only (`publishConfig` overrides + `--provenance`)
- `bun install` replaces `pnpm install` (~4-5x faster cold installs)
- `bun run` replaces `pnpm run` (~28x less startup overhead)
- `bunx` replaces `npx` in lefthook hooks
- CI workflows updated (`oven-sh/setup-bun@v2` + `actions/setup-node@v4`)
- `pnpm-lock.yaml` removed, `bun.lock` generated
- `pnpm-workspace.yaml` kept for publish compatibility
- CLI source code (`packages/cli/src/`) unchanged — shipped to end users who may not have bun

Part 5/5 of [VA-851](https://linear.app/heygen/issue/VA-851/pre-migration-configure-eslint-prettier-and-conventional-commits)

## Test plan
- [x] `bun run lint` — 0 errors
- [x] `bun run format:check` — all files pass
- [x] `bun run build` — all 5 packages build
- [x] 330 core tests pass
- [x] 18 engine tests pass
- [x] `publish.yml` unchanged (pnpm stays for npm publishing)
- [x] No `bunx`/`bun run` references in shipped source code (`packages/*/src/`)

packages/producer/package.json

@@ -22,7 +22,7 @@
     "registry": "https://registry.npmjs.org/"
   },
   "scripts": {
-    "build": "pnpm -w build:hyperframes-runtime:modular && node build.mjs",
+    "build": "bun run --cwd ../.. build:hyperframes-runtime:modular && node build.mjs",
     "typecheck": "tsc --noEmit",
     "parity:check": "tsx src/parity-harness.ts",
     "parity:fixtures": "tsx src/parity-fixtures.ts",