This PR changes `agent.play_sound_when_agent_done` from a boolean to an
enum with three options:

- `never` (default)
- `when_hidden`
- `always`

In Settings → Agent, this now appears as a _Play Sound When Agent Done_
dropdown.

Existing settings are migrated automatically:
- `false` → `never`
- `true` → `always`

### Why

A boolean only allowed the sound to be on or off. This change gives
users clearer control over when the agent notification sound should
play.

### Verification

- Added a migrator test for this setting change.
- Manually tested the settings UI, settings migration and the feature


Release Notes:

- Added new agent notification sound options

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Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>