Closes AI-344
Closes AI-342

Moves agent skills management out of the agent panel and into the
settings UI. The skill creator (previously its own crate) now lives in
`settings_ui` as a settings page, alongside the skills setup page. The
agent panel's ellipsis menu now links to the settings page instead of
hosting skill management directly.

Release Notes:

- Improved agent skills management by moving it into the settings UI

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>