TUI: expose session activity and efficiency in an inspectable stats view #201

Parent: #195

Problem

The footer provides compact status, but users cannot inspect where session time, context, model requests, and tool activity went without reading raw logs.

Scope

• Add a keyboard-friendly stats view backed only by existing canonical telemetry.
• Separate session activity, context usage, model activity, tool outcomes, and elapsed or waiting time.
• Distinguish measured values, estimates, unavailable values, and redacted values.
• Keep the default conversation uncluttered and support narrow terminals and no-color output.

Acceptance criteria

• No cost, token, latency, or success value is fabricated when the provider or runtime did not report it.
• Aggregation is deterministic across resumed sessions and does not double count retries or restored events.
• Tool names and failure summaries are secret-safe and link back to inspectable session events.
• A stable non-interactive JSON form is available for automation without ANSI formatting.
• Focused tests cover missing telemetry, retries, resume, redaction, and aggregation.
• A real terminal/tmux E2E receipt at the exact PR head demonstrates navigation, unavailable fields, narrow layout, and parity with the machine-readable result.