Refactor step-log TUI into lib/antigravity/tui.rb

Summary

The hooks-based step-log TUI pattern (permanent lines for tool calls, dynamic status line with timer, colored states, tool elapsed time) is currently duplicated in:

• examples/08_e2e_telegram_bot.rb
• examples/09_e2e_nanobanana.rb

It should live in the SDK as a reusable module.

Proposed API

# Option A: constructor flag
agent = Antigravity::Agent.new(tui: :step_log)

# Option B: explicit attachment
Antigravity::TUI::StepLog.attach(agent)

# Option C: block-based
agent.with_tui(:step_log) do
  agent.ask("hello")
end

What the TUI does

1. Step log -- permanent lines for significant WS events (tool calls, state changes, errors, first text delta)
2. Dynamic status line -- overwrites in-place with \r\e[K, shows: 🏃 running 🔧 tool_name 42s 💭💭 ⏳45s 7↕
3. Tool elapsed time -- tracks per-tool start time, logs ⏱️ run_command(...) [67s] on completion
4. Colored state -- green=running, gray=idle, red=error/cancel
5. Background ticker -- Thread ticks every 1s so timer updates even without WS traffic
6. ensure cleanup -- deactivates ticker and clears status line after ask returns

Files to create

• lib/antigravity/tui.rb -- Antigravity::TUI::StepLog class
• lib/antigravity/tui/step_log.rb -- if we want a directory structure
• Extract from examples/09_e2e_nanobanana.rb (the reference implementation)

Dependencies

• Requires the hooks system (Antigravity::Hooks with on/emit)
• Uses :ws_message event from conversation.rb

Priority

P3 -- nice to have, current examples work fine with inline code.

Start with the reference implementation in examples/09_e2e_nanobanana.rb, then compare the duplicated pattern in examples/08_e2e_telegram_bot.rb. Read the Antigravity::Hooks on/emit system and the :ws_message flow in conversation.rb before choosing among the proposed APIs. Done means the behavior lives in lib/antigravity/tui.rb or lib/antigravity/tui/step_log.rb and both examples can use the reusable TUI with ticker cleanup after ask returns.