Refine Plugins and Skills sidebar icons

[brsbl]

Human comments

What was wrong

The Plugins row used a block-game glyph and the Skills row used an open book, which did not match the requested grid and closed-book visual language.

What changed

• Changed Plugins to the existing GridView semantic icon, backed by GridViewIcon.
• Changed Skills to the existing closed-book Explore semantic icon, backed by Book02Icon.
• Updated the focused sidebar icon assertions.
• Preserved row placement, hover treatment, navigation, visibility, and active-state behavior.

Before

After

How you verified

• Chrome for Testing 151.0.7922.71 at 1440×900 on exact parent head e44b97e27 and exact PR head d2c8f18c5: hard-reloaded each revision and captured the matching sidebar evidence above.
• Confirmed the root sidebar renders Plugins with GridView, Skills with the closed Book02Icon, and Automations with CalendarSync after plugin registration settles.
• Hovered the Plugins row and confirmed its existing hover treatment remains intact with no browser console or runtime errors.
• Remote CI passed: build, typecheck, lint, app/server/package/integration tests, Linux and macOS package smoke, and version lockstep checks.

BB-Thread-ID: thr_czdxmmyenv

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