Add expandable plugin sidebar sub-items

[MacHatter1]

Human comments

What was wrong

Plugin navigation panels could register only one flat sidebar destination, so a plugin with several related views had to consume multiple top-level rows or build navigation inside its panel. The app and SDK had no contract for child destinations, active-route selection, or expandable sidebar presentation.

What changed

• Added the experimental experimental_sidebarSubItems plugin navigation contract, including child paths, icons, and sidebar accessories while preserving existing top-level experimental_sidebarAccessory behavior.
• Collected, validated, cached, and rendered expandable plugin sidebar children with nested-route active matching, accessible controls, active-child expansion, and user-controlled collapsing.
• Added coverage across the SDK harness, app chrome, sidebar interactions, API map, and the plugin API testing plugin.
• Documented the experimental API and bumped the Plugin SDK compatibility version to 0.4.30.
• No host-daemon wire, CLI, or configuration change is involved, so HOST_DAEMON_PROTOCOL_VERSION is unchanged.

How you verified

• pnpm exec turbo run typecheck --force --concurrency=4 --filter=@bb/app --filter=@bb/server --filter=@get-bb/plugin-sdk --filter=@bb/plugin-api-map --filter=bb-plugin-plugin-api-tester
• pnpm exec turbo run build --force --concurrency=4 --filter=@bb/app --filter=@bb/server --filter=@get-bb/plugin-sdk --filter=@bb/plugin-api-map --filter=bb-plugin-plugin-api-tester
• Focused Turbo tests for PluginNavSidebarItems, plugin nav-panel chrome, the Plugin SDK app harness, plugin authoring docs, the API map, and bb-plugin-plugin-api-tester.
• Loaded the testing plugin in the development app and exercised child icons, accessories, route activation, expansion, and collapsing.

Fixes: N/A

AGENT GENERATED