Improve Tools Hub filtering and plugin navigation

[brsbl]

Improves the Tools Hub follow-up experience without changing its default-off experiment gate or the detail-page contract from #856.

What changed

• Skills Library now opens filtered to bb skills and adds a Source filter for built-in and plugin-bundled skills.
• Plugin-bundled skills show their owning plugin on overview rows; their existing detail-page provenance remains intact.
• Plugins adds an all-versus-built-in Source filter while continuing to default to all installed plugins.
• Plugin capability rows now explain their purpose. Skill capabilities use SKILL.md frontmatter descriptions, and app surfaces, settings, services, schedules, and thread integrations no longer fall back to implementation-category labels alone.
• With Tools Hub enabled, Settings hides only the legacy plugin-management entry. The separate per-plugin settings group stays visible, plugin settings deep links remain valid, and plugin detail pages link across to Settings.

The Settings bug came from applying the Tools Hub flag to both the static management entry and the dynamically registered plugin settings entries even though they are separate user-facing surfaces.

Validation

• pnpm exec turbo run build typecheck lint — 68/68 tasks successful
• Focused app tests — 61 passed, including the 69/21/3 public-export guard and detail-page recipe contract
• Focused server plugin tests — 34 passed
• Live desktop QA with checkout-scoped data for Skills, Plugins, capability details, and both Settings plugin surfaces

Scope boundary

Backlog item 6 is intentionally not included. The Automations-to-Schedules and Plugins & Skills restructure should be evaluated as a separate PR because it changes routes, sidebar structure, breadcrumbs, and the automations plugin.