Fix diagnostics scrolling and list data visibility

[luoyuctl]

Summary

• route the Diagnostics tab through the existing Bubble viewport so PageDown/j/k/scroll keys work
• keep Session List table rows visible on compact 80x24 terminals by hiding summary panels when vertical space is tight
• widen 150-169 column layouts so COST/TOKENS/DURATION/ANOM/HEALTH/ISSUE render real values instead of clipped placeholders

Real local verification

• Built /tmp/agenttrace-tui-fix from this branch and ran it against the real local 1714-session dataset
• 80x24 Session List now shows table rows immediately, including COST/TOKENS/HEALTH values like $25.2403, 31.8M, 88%
• Diagnostics PageDown changed the view from Scroll: 0% to Scroll: 11% and revealed later slow-path content
• 160x36 Session List shows COST/TOKENS/DURATION/ANOM/HEALTH/ISSUE row values, including full No major anomaly
• agenttrace --overview -f json reports 1714 sessions; agenttrace --doctor -f json reports 1714 sessions with SQLite-backed DB sources included

Tests

• go test ./internal/tui -run 'Test(DiagnosticsViewUsesScrollableViewport|CompactHeightListKeepsTableRowsVisible|WideListKeepsFullOperationalColumns)' -count=1 -v
• go build -o /tmp/agenttrace-tui-fix ./cmd/agenttrace
• go test ./...

Dependency note

No new dependency was needed; the repo already had github.com/charmbracelet/bubbles/viewport, which covers the scrolling behavior cleanly.