ux(vscode): make Task Hub the primary workflow router and simplify sidebar information architecture

[oaslananka]

Summary

Promote the completed task-oriented command work from #498 into the primary day-to-day navigation model and reduce duplicate or ambiguous sidebar/setup surfaces.

Evidence

A live 2026-08-30 VS Code audit confirmed that Task Hub provides clear Review, Validate, Fabrication Release, Automate, and Maintain entry points. At the same time, the primary sidebar exposes Project, Validation, Quality Gates, BOM, Netlist, Variants, Library, DRC Rules, Component Search, and MCP & Tools concurrently.

Additional friction observed:

• MCP setup/install actions appear in multiple surfaces.
• BOM and Netlist views can render completely empty without explaining why or how to populate them.
• Advanced integration capabilities compete visually with the core project/validate/release workflow.

Scope

• Treat Task Hub as the main workflow router for high-level user intent.
• Keep sidebar views contextual and evidence-oriented rather than duplicating setup/action catalogs.
• Consolidate repeated MCP onboarding actions into one authoritative flow.
• Add useful empty/loading/error states to BOM, Netlist, and other data-dependent views.
• Preserve direct command access and existing command IDs.

Acceptance criteria

• Review, Validate, Release, Automate, and Maintain have one canonical primary entry point each.
• Sidebar sections expose state/results and only the most relevant contextual actions.
• MCP install/setup/reconnect actions do not produce competing onboarding flows.
• Empty BOM/Netlist states explain prerequisites and offer a safe next action where appropriate.
• Advanced/library/import/MCP capabilities remain discoverable through progressive disclosure.
• Existing commands remain callable and backward compatible unless separately deprecated.
• Keyboard-only and screen-reader navigation remains complete.

Validation

• Command-surface policy and context-key tests.
• Webview/tree-view accessibility tests.
• Visual regression coverage for narrow sidebar and secondary-sidebar-open layouts.
• A documented first-use path from opening a KiCad project to Review/Validate/Release.