`/plan-eng-review` Runs Produce Multiple Unreconciled Task Sets

## Summary

When /plan-eng-review is run more than once on the same branch, it creates new task and plan files without reading what
already exists. The files look like this:
```
tasks-eng-review-.jsonl
tasks-eng-review-.jsonl
```

The second run has no awareness of the first. Tasks may conflict, or re-plan work that's already been
completed. I did not see a way to execute against both files with confidence that they don't conflict.

## Steps to reproduce

1. Run /plan-eng-review on a branch → observe files: tasks-eng-review-T1.jsonl + *-test-plan-T1.md
2. Make progress, add context, run again → observe new files: tasks-eng-review-T2.jsonl + *-test-plan-T2.md
3. Both files coexist. T2 was generated without reading T1. Tasks may overlap, contradict, or re-plan completed work.

## Expected behavior

When a prior plan exists for the current branch, invoking `/plan-eng-review` should notify the user and offer to produce a single reconciled output — one task list that carries forward incomplete work, drops completed tasks, and resolves any conflicts with new findings from the current pass. The re-run should incorporate and/or replace the prior plan.

**Secondary issue**: tasks-eng-review-*.jsonl filenames omit the branch name (unlike *-eng-review-test-plan-*.md which includes it), making branch-scoped filtering impossible on the tasks files without reading their contents.

## Related Issues

#1343