[ARC/DinD] Emit chroot.binariesSourcePath and chroot.identity in stdin-config for DinD topology

## Problem

AWF gained `chroot.binariesSourcePath` and `chroot.identity.*` config support in [gh-aw-firewall#4587](https://github.com/github/gh-aw-firewall/pull/4587), which would allow the copilot binary and runner identity to be properly resolved without workflow-level workarounds. However, gh-aw does not emit these fields in the stdin-config passed to AWF.

This forces ARC/DinD users to maintain a bootstrap action that:
1. Copies the copilot binary into the DinD daemon's `/usr/local/bin/` via `docker run`
2. Creates a shim wrapper to set HOME/USER/LOGNAME env vars (because AWF chroot passes the wrong identity vars)
3. Pre-seeds the DinD daemon's `/etc/passwd` with the runner UID

## Expected Behavior

When gh-aw detects an ARC/DinD topology (e.g., `DOCKER_HOST=tcp://...` or a DinD sidecar pod), it should emit:

```json
{
"chroot": {
"binariesSourcePath": "/path/to/staged/binaries",
"identity": {
"user": "runner",
"uid": 1001,
"gid": 1001,
"home": "/tmp/gh-aw/home"
}
}
}
```

This would let AWF handle binary staging and identity resolution natively, eliminating the bootstrap action workaround entirely.

## Context

- Upstream report: https://github.com/github/gh-aw/issues/34896
- AWF implementation: [gh-aw-firewall#4587](https://github.com/github/gh-aw-firewall/pull/4587)
- AWF tracking issues: [gh-aw-firewall#4829](https://github.com/github/gh-aw-firewall/issues/4829), [gh-aw-firewall#4830](https://github.com/github/gh-aw-firewall/issues/4830)
- Current workaround: `arc-gaw-bootstrap` composite action (see issue #34896 comments from @github-antoine-brechon, June 12 2026)
- gh-aw version tested: v0.79.5
- AWF version tested: v0.27.1

## Impact

Eliminating this workaround would:
- Remove ~80 lines of bootstrap shell script from consumer repos
- Remove the need to bake node into a custom DinD image
- Remove the copilot shim wrapper (HOME/USER/LOGNAME override)
- Make ARC/DinD a first-class supported topology