fix(tui): show usage and team roster in the collapsed sidebar band from startup

[Sayt-0]

With the sidebar rendered as a band (sidebar_position: top or bottom), two pieces of information shown by the vertical sidebar were missing:

Issue	Cause	Fix
Context/cost absent at startup, only appearing after the first message	tokenUsageSummary() returned "" while sessionUsage was empty, unlike the vertical Token Usage tab which always renders	The band renders the same usage line unconditionally (◉ 0 $0.00 before any usage); the hide-usage setting is still honored
Team roster not visible when running a multi-agent config	agentSummaryCollapsed() showed only the current agent plus a bare +N count	The band lists every team agent by name in its accent color, after the current agent and its model

Band rendering, before

 New session
 █ ~/repo (main)
 ▶ root openai/gpt-3.5-turbo +2 · █ 1 tool

Band rendering, after

 New session
 █ ~/repo (main)                                  ◉ 0 $0.00
 ▶ root openai/gpt-3.5-turbo · planner · reviewer · █ 1 tool

Notes

• CollapsedViewModel.LineCount and RenderCollapsedView derive from the same view model, so the band height accounts for the extra content (including wrapping) with no layout change.
• Behavior verified end to end with the tuitest harness (top sidebar, simple agent and team) at startup.

Tests

• TestTokenUsageSummary_Empty replaced by TestTokenUsageSummary_NoUsageYet_ShowsZeroLine (the band now shows the zero line).
• New TestCollapsedInfoLine_ShowsTeamRoster pins the roster names in the band and the removal of the +N count.