feat: display filter metrics in TUI dashboard

[bug-ops]

Parent

Epic #446 (M26.1), depends on #442 (filter metrics)

Problem

Filter metrics (savings %, confidence distribution, commands filtered) are only logged to tracing. TUI dashboard should display them in real-time.

Design

• Expose FilterMetrics from OutputFilterRegistry via a method or watch channel
• Forward metrics to TUI via AgentEvent (similar to how tool events are forwarded)
• Display in a TUI widget: total savings %, commands filtered, confidence breakdown

Acceptance Criteria

• FilterMetrics accessible from registry
• Metrics forwarded to TUI via AgentEvent or MetricsCollector
• TUI widget displays: savings %, filtered/total commands, confidence distribution
• Updates in real-time as commands are filtered