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/`)

.github/workflows/release.yml

@@ -18,17 +18,14 @@ jobs:
       pull-requests: write
     steps:
       - uses: actions/checkout@v4
-      - uses: pnpm/action-setup@v4
-        with:
-          version: 10
+      - uses: oven-sh/setup-bun@v2
       - uses: actions/setup-node@v4
         with:
           node-version: 22
-          cache: pnpm
-      - run: pnpm install --frozen-lockfile
+      - run: bun install --frozen-lockfile
 
       - name: Bump versions
-        run: pnpm set-version ${{ inputs.version }}
+        run: bun run set-version ${{ inputs.version }}
 
       - name: Create release PR
         env: