Hides the legacy Rules surfaces when the `skills` feature flag is
enabled, since Rules are surfaced through the Skills UI in that case:

- The **Rules** entry in the agent panel's ellipsis (triple-dot)
dropdown menu is hidden.
- The `assistant: open rules library` command palette action is hidden.

The `@rules` autocomplete in the chat input is intentionally left alone
for now.

### Implementation notes

`cx.has_flag::<SkillsFeatureFlag>()` reads from a global feature-flag
store that is populated asynchronously from the server:

- **Command palette**: `update_command_palette_filter` is already re-run
via `cx.on_flags_ready` once flags arrive, so the filter is reapplied
with the correct value. No new wiring needed.
- **Dropdown menu**: the popover's menu closure runs on every open, so
the latest flag value is reflected at click time.

Release Notes:

- N/A