Redesign assistant workspace controls

[jorisjonkers-dev-agents]

Summary

• move assistant agent launch controls back into the workspace top header
• restore live sessions as horizontally scrollable top tabs and drop stopped/failed sessions from the strip
• make the right workspace sidebar a foldable overlay while preserving stage, repository attach, and split guidance flows

Commits

• e002e40 assistant-ui: compact agent picker for header
• 6c6a975 assistant-ui: make session tabs scroll horizontally
• b3ab367 assistant-ui: restore top workspace controls

Verification

• corepack pnpm typecheck
• corepack pnpm lint
• corepack pnpm test
• corepack pnpm build
• corepack pnpm depcruise