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/context_usage.go

@@ -0,0 +1,78 @@
+package agent
+
+import (
+	"github.com/sipeed/picoclaw/pkg/bus"
+)
+
+// computeContextUsage estimates current context window consumption for the
+// given agent and session. Includes history, system prompt (with dynamic context,
+// summary, and skills — mirroring BuildMessages composition), and tool definitions.
+// The output reserve (MaxTokens) is not counted as "used" but reduces the
+// effective budget, matching isOverContextBudget's compression trigger:
+//
+//	compress when: history + system + tools + maxTokens > contextWindow
+//	equivalent to: history + system + tools > contextWindow - maxTokens
+//
+// Returns nil when the agent or session is unavailable.
+func computeContextUsage(agent *AgentInstance, sessionKey string) *bus.ContextUsage {
+	if agent == nil || agent.Sessions == nil {
+		return nil
+	}
+	contextWindow := agent.ContextWindow
+	if contextWindow <= 0 {
+		return nil
+	}
+
+	// History tokens
+	history := agent.Sessions.GetHistory(sessionKey)
+	historyTokens := 0
+	for _, m := range history {
+		historyTokens += EstimateMessageTokens(m)
+	}
+
+	// System message tokens: uses EstimateSystemTokens which mirrors
+	// the full system message composition in BuildMessages (static prompt,
+	// dynamic context, active skills, summary with wrapping prefix).
+	systemTokens := 0
+	if agent.ContextBuilder != nil {
+		summary := agent.Sessions.GetSummary(sessionKey)
+		// Pass nil for active skills: skills are only injected when the user
+		// explicitly activates them via /use, which is rare. Using nil matches
+		// the common case and avoids over-counting all installed skills.
+		systemTokens = agent.ContextBuilder.EstimateSystemTokens(summary, nil)
+	}
+
+	// Tool definition tokens
+	toolTokens := 0
+	if agent.Tools != nil {
+		toolTokens = EstimateToolDefsTokens(agent.Tools.ToProviderDefs())
+	}
+
+	// Used = history + system (includes summary) + tools
+	usedTokens := historyTokens + systemTokens + toolTokens
+
+	// Effective budget = contextWindow minus output reserve (maxTokens)
+	effectiveWindow := contextWindow - agent.MaxTokens
+	if effectiveWindow < 0 {
+		effectiveWindow = contextWindow
+	}
+
+	// compressAt = effectiveWindow: aligns with isOverContextBudget's
+	// proactive trigger (msgTokens + toolTokens + maxTokens > contextWindow).
+	compressAt := effectiveWindow
+
+	usedPercent := 0
+	if compressAt > 0 {
+		usedPercent = usedTokens * 100 / compressAt
+	}
+	if usedPercent > 100 {
+		usedPercent = 100
+	}
+
+	return &bus.ContextUsage{
+		UsedTokens:       usedTokens,
+		TotalTokens:      contextWindow,
+		CompressAtTokens: compressAt,
+		UsedPercent:      usedPercent,
+	}
+}