CSS design for agents web surface

Add CSS for the agents web surface to the shared design system.

New CSS Classes (added to agent-specific stylesheet, like pipeline has pipelineCss)

Agent Cards

• .ag-stats — Stats bar (like .pl-stats)
• .ag-stat, .ag-stat-value, .ag-stat-label — Individual stat boxes
• .ag-runs — Agent list container
• .ag-run — Individual agent card
• .ag-run-active — Running/idle agent (blue/yellow left border)
• .ag-run-header — Card header with ID, title, badges
• .ag-run-meta — Timestamp, cost, duration
• .ag-run-prompt — Prompt preview text

Status Badges

• .ag-badge-running — Blue, pulsing
• .ag-badge-idle — Yellow/amber (distinct from running — agent is waiting for input)
• .ag-badge-completed — Green
• .ag-badge-failed — Red
• .ag-badge-stopped — Gray

Event Stream

• .ag-events — Event container
• .ag-ev — Individual event
• .ag-ev-thinking — Thinking block (collapsible)
• .ag-ev-tool — Tool call
• .ag-ev-result — Tool result (collapsible)
• .ag-ev-text — Assistant text output
• .ag-ev-turn — Turn boundary
• .ag-ev-error — Error
• .ag-ev-steering — Steering message

Interaction

• .ag-input — Message input area
• .ag-send-btn — Send button
• .ag-actions — Action bar

Diff View

• .ag-diff — Diff container
• .ag-diff-add — Added line (green)
• .ag-diff-del — Deleted line (red)
• .ag-diff-hunk — Hunk header (blue)

All colors should use CSS custom properties (var(--c-*)) for light/dark mode support.