[Bug]: jira workitem update --parent reports success but does not write the field

### TWG CLI version

1.2.7

### Operating system

Ubuntu 24.04 (WSL2)

### Command or agent request

Given an epic `ABC-1` and a task `ABC-2` whose `parent` is empty:

```bash
twg jira workitem update --id ABC-2 --parent ABC-1 -o json
twg jira workitem get ABC-2 --fields parent -o json
```

### Expected behavior

Either the parent is set, or the command reports that it was not.

### Actual behavior

The update reports success for the `parent` field specifically, and the field is unchanged:

```
data.results: [{"alias": "update_parent", "fieldId": "parent", "success": true}]
data.success: true
exit code: 0
```

```
fields.parent: null
```

The issue changelog records no `parent` change either, so this is not read-after-write lag: nothing was written.

The same field, same value, same subcommand, through the generic field flag, does write:

```bash
twg jira workitem update --id ABC-2 --fields-json '{"parent":{"key":"ABC-1"}}' -o json
```

After this call `fields.parent` is `ABC-1`, and the changelog contains exactly one parent entry, timestamped at this call rather than at the earlier `--parent` one.

The two calls are indistinguishable from their output: the envelopes are identical, `results[].success: true` included. There is no field a caller can inspect to tell the write that happened from the write that did not, which makes this harder to detect than an error would be. Re-reading the field is the only way.

Observed on 7 issues across two days on the same version, all of them silent in the same way.

Impact: an agent reorganising an issue hierarchy gets a success for every call and changes nothing. A caller that trusts the exit code, or even the per-field `success` flag, reports work it did not do.

Workaround: use `--fields-json '{"parent":{"key":"..."}}'` on the same `workitem update`, and re-read the field afterwards.

Start by reproducing the issue with `twg jira workitem update --id ABC-2 --parent ABC-1 -o json`, then verify the result with `twg jira workitem get ABC-2 --fields parent -o json` and compare it with the working `--fields-json` command. Trace the `workitem update --parent` path and its `update_parent` result reporting. Done means `--parent` writes the parent or reports failure accurately, with coverage for the command and its success response.

Written by the indexing model from the issue text.