Add a `bun_version` input to avoid a duplicate setup-bun step

The action installs Bun itself, but pins the version (`1.3.14`). Consumers who need a different version must run `oven-sh/setup-bun` themselves and pass `path_to_bun_executable`, so Bun is set up twice in the workflow.

A `bun_version` input passed through to the action's own `setup-bun` step would remove the duplicate setup:

```yaml
- uses: anthropics/claude-code-action@v1
with:
bun_version: 1.4.0
```

This matters when a repo pins its runtime — `devEngines` with `onFail: "error"` rejects any other version, so the action's Bun breaks every command Claude runs.

It would also help #618: one action reference instead of two, so `oven-sh/setup-bun` no longer needs separate allowlisting.