Removes the `skills` feature flag now that Skills are ready to ship to
all users. Cleans up all `cx.has_flag::<SkillsFeatureFlag>()` gates
across the agent, agent_ui, prompt_store, and title_bar crates, drops
the now-unused `feature_flags` dependency from `prompt_store` and
`title_bar`, and removes the dead `update_flags(true, vec!["skills"])`
calls from the agent's skills tests.

Closes AI-269

Release Notes:

- Enabled Skills for all users.

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Co-authored-by: MartinYe1234 <52641447+MartinYe1234@users.noreply.github.com>
Co-authored-by: Martin Ye <martinye022@gmail.com>