feat: add configurable context usage display

[kidush]

Summary

Adds a configurable context usage display for Tau’s TUI.

Users can now choose how the compact context meter below the prompt is shown via ~/.tau/tui.json:

{
  "context_usage_display": "both"
}

Supported values:

• tokens - current/default behavior
• percent - show percentage of the active model context window
• both - show tokens and percentage
• off - hide the compact context meter

Changes

• Added context_usage_percent(...) in tau_coding.context_window.
• Exposed CodingSession.context_usage_percent.
• Added TuiSettings.context_usage_display.
• Wired the setting into compact TUI session info rendering.
• Updated /session to report context usage as tokens and percentage.
• Updated configuration docs.
• Added a dev note with the implementation plan.

Checks

uv run pytest tests/test_context_window.py tests/test_coding_session.py tests/test_tui_config.py tests/test_tui_app.py tests/test_commands.py -q
uv run ruff check .
uv run ruff format --check .

All checks passed.