feat(agent): add context usage ring indicator and /context command (#2537)

Add a context window usage indicator to the web chat UI and a /context
slash command that works across all channels.

Backend:
- Add computeContextUsage() estimating history + system + tool tokens
- Attach ContextUsage to outbound messages via the pico WebSocket protocol
- Add /context command showing context stats as formatted text
- Add EstimateSystemTokens() on ContextBuilder for system prompt estimation

Frontend:
- Add ContextUsageRing component (SVG ring + hover/tap popover)
- Show usage percentage, token counts, and compression threshold
- Hover on desktop (150ms leave delay), tap on mobile
- "View Details" sends /context with 1s cooldown
- i18n support (en/zh) for popover labels

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/agent_command.go

@@ -214,6 +214,24 @@ func (al *AgentLoop) buildCommandsRuntime(
 		rt.AskSideQuestion = func(ctx context.Context, question string) (string, error) {
 			return al.askSideQuestion(ctx, agent, opts, question)
 		}
+
+		rt.GetContextStats = func() *commands.ContextStats {
+			if opts == nil || agent.Sessions == nil {
+				return nil
+			}
+			usage := computeContextUsage(agent, opts.SessionKey)
+			if usage == nil {
+				return nil
+			}
+			history := agent.Sessions.GetHistory(opts.SessionKey)
+			return &commands.ContextStats{
+				UsedTokens:       usage.UsedTokens,
+				TotalTokens:      usage.TotalTokens,
+				CompressAtTokens: usage.CompressAtTokens,
+				UsedPercent:      usage.UsedPercent,
+				MessageCount:     len(history),
+			}
+		}
 	}
 	return rt
 }