Feature Request: Preserve start field on completed tasks when using Timewarrior integration

**Summary:**
When Taskwarrior is integrated with Timewarrior (via `on-modify.timewarrior` hook), starting a task correctly sets the `start` timestamp. However, completing the task (`task done`) deletes the `start` field.
This makes it impossible to export complete start-end timing data from Taskwarrior alone after task completion.

**Problem:**
- After `task start`, the `start` field is set.
- After `task done`, the `start` field is deleted, leaving only the `end`.
- Users lose direct access to the start timestamp unless they consult Timewarrior separately.

**Suggestion:**
Introduce a configuration option, e.g.:

```bash
rc.preserve_start_on_done=yes
```

- If enabled, Taskwarrior would **retain the `start` field** even after task completion.
- Default behavior (deleting `start`) would remain unchanged if the setting is `no` or absent.

**Why important:**
- Easier post-analysis (start/end/duration) directly from Taskwarrior exports.
- Simpler scripts, reports, or external integrations without needing Timewarrior parsing.
- Full lifecycle visibility in Taskwarrior alone.

**Environment:**
- Taskwarrior: 2.6.1
- Timewarrior: 1.4.3
- Platform: WSL by Ubuntu 22.04.3 LTS

**Notes:**
- Timewarrior handles time tracking well, but users combining both Taskwarrior and Timewarrior would benefit from a persistent `start` field in Taskwarrior's own data model.