When users have the `multi-workspace` feature flag enabled (agent-v2),
the default dock position for the agent panel is changed from right to
left.
This adds a `update_default_settings` method to `SettingsStore` that
allows mutating defaults in place, then observes the
`AgentV2FeatureFlag` to update the agent dock default accordingly.
Because this modifies the defaults layer, user-configured dock positions
still take precedence.
Closes AI-15
(No release notes because multi-agent is feature-flagged.)
Release Notes:
- N/A