[dependabot:release-train-updater] create_pull_request safe-output tool fails with "No changes to commit" despite verified clean commits (systemic, reproduced 5+ t

## Summary

The `create_pull_request` safe-output tool consistently fails with the error:

```
No changes to commit - no commits found
```

despite the local git state being verifiably correct: a clean working tree, exactly one commit ahead of `origin/main`, and a non-empty diff containing only the intended files.

## Reproduction history

This has now been reproduced across **5+ independent workflow runs** on `githubnext/gh-aw-cao`, spanning at least two distinct dependency-update candidates and many different branch names/commit SHAs:

- `@github/copilot-sdk` version bump (multiple runs, 2026-09-08)
- `astro` 7.3.1→7.3.2 + `@astrojs/markdown-remark` 7.3.0→7.3.1 (runs at 2026-09-08 17:35, 18:56, 19:32, and this run)

Each time:
1. `package.json` + `package-lock.json` were updated cleanly (2 files only).
2. A new branch was created off the latest `origin/main` tip.
3. `git add` + `git commit` succeeded with a real, non-empty diff.
4. `git log origin/main..HEAD` showed exactly 1 commit ahead, clean tree.
5. `create_pull_request` was called with that branch name (matching `git branch --show-current` exactly) — and failed with "No changes to commit - no commits found".
6. Renaming the branch and retrying once more reproduced the identical failure.

This run (2026-09-08 20:30 UTC), branch `dependabot-agent/npm-astro-7.3.2-markdown-remark-7.3.1-attempt5-1788899699`, commit `080d8c92`, is the 5th independent confirmation.

## Impact

The dependency-release-train worker cannot land any dependency PRs at all via `create_pull_request` in this repository/environment, blocking all automated dependency maintenance through this path. Every run since 2026-09-08 has ended in `report_incomplete` for this reason.

## Suggested next step

Investigate the `create_pull_request` safe-output handler (branch/commit detection logic, possibly related to how it diffs against the target base, working-directory state at tool-invocation time, or how `git status`/`git log` is evaluated by the handler versus the agent's shell). Until fixed, consider having the release-train worker prefer `add_comment`/`create_issue` outputs over `create_pull_request` for this repository, or use `push_to_pull_request_branch` against a manually pre-created PR as a workaround if feasible.

## Control Plane
- Correlation ID: 34274301438-143
- Central repo: githubnext/gh-aw-cao
- Run URL: https://github.com/githubnext/gh-aw-cao/actions/runs/34274301438

> [!WARNING]
>
> Firewall blocked 4 domains
>
> The following domains were blocked by the firewall during workflow execution:
>
> - `msfeed12.pkgs.visualstudio.com`
> - `msfeed17.pkgs.visualstudio.com`
> - `msfeed2.pkgs.visualstudio.com`
> - `msfeed25.pkgs.visualstudio.com`
>
> To allow these domains, add them to the `network.allowed` list in your workflow frontmatter:
>
> ```yaml
> network:
> allowed:
> - defaults
> - "msfeed12.pkgs.visualstudio.com"
> - "msfeed17.pkgs.visualstudio.com"
> - "msfeed2.pkgs.visualstudio.com"
> - "msfeed25.pkgs.visualstudio.com"
> ```
>
> See [Network Configuration](https://github.github.com/gh-aw/reference/network/) for more information.
>
>

> Generated by [:dependabot: Dependabot / Release Trains](https://github.com/githubnext/gh-aw-cao/actions/runs/34274773737) · copilot · auto · 73.2 AIC · ⌖ 12.8 AIC · ⊞ 20.5K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-cao+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fgh-aw-cao%2Fdependabot-release-train-updater%22&type=issues)
>

_{Add this agentic workflow to your repo}

To install this agentic workflow, run

```
gh aw add githubnext/gh-aw-cao/.github/workflows/dependabot-release-train-updater.md@main
```

> - [x] expires on Sep 22, 2026, 8:38 PM UTC

Start at the create_pull_request safe-output handler; no source file or test is named, so trace its branch and commit detection against the reported workflow invocation. Reproduce with a clean branch exactly one commit ahead of origin/main, then verify done when the dependency-release-train workflow can create the pull request successfully.

Written by the indexing model from the issue text.