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, e.g. 12k/200k
• percent — percentage of the active model context window, e.g. 6%
• both — tokens and percentage, e.g. 12k/200k · 6%
• off — hide the compact context meter entirely

Rendered output follows the footer convention from #407: the meter sits on its own line below provider:model, with no context label. In off mode that line is omitted rather than left blank.

Changes

• Added context_usage_percent(...) in tau_coding.context_window.
• Exposed CodingSession.context_usage_percent.
• Added TuiSettings.context_usage_display, defaulting to tokens; missing or invalid values in an existing tui.json fall back to tokens / raise TuiConfigError.
• Wired the setting into compact TUI session info rendering, including the render-cache fingerprint added in Improve TUI sidebar session insights #400 so the footer redraws when the setting changes.
• Updated /session to report context usage as tokens and percentage. This output is deliberately not controlled by context_usage_display, which only governs TUI chrome.
• Updated configuration docs.
• Added a dev note with the implementation plan.

Checks

uv run ruff check .
uv run ruff format --check .
uv run mypy
uv run pytest -q

All passing — 1062 tests.

Rebased onto main @ 102482b.