feat: token savings tracking and metrics

[bug-ops]

Parent

Epic: #426

Summary

Track token savings from output filtering and expose via metrics for monitoring and tuning.

Design

• Extend FilterResult with raw_chars and filtered_chars
• Token estimation: chars / 4 (standard heuristic)
• Record per-filter stats in MetricsCollector:
  • tool_output_raw_tokens (counter)
  • tool_output_filtered_tokens (counter)
  • tool_output_savings_pct (gauge, rolling average)
  • tool_output_filter_hits (counter per filter name)
• Surface in TUI dashboard (tools panel)
• Log summary at session end: "Tool output filtering saved ~X tokens (Y%)"

Acceptance Criteria

• FilterResult carries raw/filtered sizes
• MetricsCollector updated per tool execution
• Session-end summary logged
• TUI display (optional, if TUI feature enabled)
• Unit tests for metric recording