feat(workspace): add multi-project workspace switching and hub mode

[howznguyen]

Description

• Add global project registry (~/.knowns/registry.json) with scan/add/remove
• Add storage.Manager for thread-safe runtime project switching
• Add /api/workspaces routes (list, switch, scan, auto-scan, delete)
• Add WorkspacePicker UI component with auto-scan support
• Add user prefs store and /api/user-prefs routes
• Add OpenCode daemon integration for agent platform support
• Refactor DocsPage into composable sub-views (header, create, highlight)
• Extract MDRender into focused editor utilities (mentions, headings, badges)
• Add doc line/range/heading anchor reference syntax to KNOWNS.md
• Sync Kiro skills platform alongside agent/claude skill copies

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Other (please describe):

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published