fix(repo-assist): remove title-prefix restriction from push-to-pull-request-branch

## Problem

The `repo-assist` workflow has `title-prefix: "[Repo Assist] "` set on its `push-to-pull-request-branch` safe output:

```yaml
safe-outputs:
push-to-pull-request-branch:
target: "*"
title-prefix: "[Repo Assist] "
max: 4
```

This configuration is self-contradictory. `target: "*"` says "push to any PR", but `title-prefix: "[Repo Assist] "` rejects every push to a PR whose title doesn't start with `[Repo Assist] ` — which is every PR not originally created by Repo Assist.

## Impact

When a user invokes `/repo-assist` to fix CI failures, address review comments, or update an open PR created by a human or another workflow, the agent correctly identifies the changes needed, generates the patch, and queues a `push_to_pull_request_branch` operation. The safe_outputs job then silently rejects it with:

```
✗ Message 1 (push_to_pull_request_branch) failed: Pull request title "[other-workflow] ..."
does not start with required prefix "[Repo Assist] "
```

The user gets no useful feedback, and the fix is never applied.

## Root Cause

`title-prefix` on `push-to-pull-request-branch` was likely copied from `create-pull-request` where it makes sense (to namespace PRs Repo Assist creates). On `push-to-pull-request-branch` it has the opposite effect — it prevents Repo Assist from pushing to PRs it *didn't* create, which is precisely the on-demand use case.

## Fix

Remove `title-prefix` from `push-to-pull-request-branch` in `workflows/repo-assist.md`:

```yaml
safe-outputs:
push-to-pull-request-branch:
target: "*"
max: 4
```

`title-prefix` should remain on `create-pull-request` where it namespaces new PRs correctly. The `max: 4` cap and existing `protected-files` guardrails already provide the right safety boundary for pushes.

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*Issue filed by Claude (Anthropic)*