feat: embed AgentTeams activity panel in Better Sidebar tab

[n-pll]

Summary

Embed the AgentTeams activity panel as a DSH Better Sidebar tab when dsh-better-sidebar is present, instead of mounting the fixed top-right floater.

Changes

• Add an optional �mbedded prop to ActivityPanel:
  • forces the expanded panel view
  • skips the page-level data-agent-teams-panel-open layout shift
  • hides the floating close button
• Register an �gent-teams tab through ctx.get('betterSidebar').registerTab(...) when Better Sidebar is available, and route the conversation card's OPEN_PANEL_EVENT to �etterSidebar.openTab(...).
• Add .panel.embedded CSS so the panel fills the sidebar tab instead of using position: fixed.
• Declare dsh-better-sidebar as an optional peer dependency and dev dependency.

Behavior

• With Better Sidebar installed: the activity panel becomes a single-instance "Agent Teams" sidebar tab, and the card button opens that tab.
• Without Better Sidebar: the existing body-portal floater remains the fallback, so behavior is unchanged.

Verification

• pnpm install, pnpm build, and pnpm typecheck pass.
• Tested against a DSH web profile: the served client bundle contains the new tab registration and embedded panel markup.