Add TUI dirty-flag to skip unconditional 250ms redraws

## Context

`tui_loop()` calls `terminal.draw()` every 250ms tick including idle. Widget rendering and diff still consume CPU.

## Solution

Add `dirty: bool` flag set by event handlers, skip draw when `!dirty` and no agent events.

**Epic:** #862 | **Effort:** S | **Crate:** zeph-tui