feat(tui): command palette, diff viewer, dedicated patch slot

[Chukwuebuka-2003]

Implements three TUI polish features from the roadmap: a searchable command palette modal, a syntax-highlighted diff viewer modal, configurable keybindings for both new features, and a dedicated patch storage slot that replaces heuristic text scanning.

Changes

Command Palette (Ctrl+P)

• New CommandPaletteScreen modal (ModalScreen[str]) with a search Input filter and ListView of all registered SlashCommand objects
• Selecting a command inserts its name into the prompt
• Replaces the old action_open_command_palette that simply typed / into the input
• All commands from _session_command_registry() are available
  Diff Viewer (Ctrl+L)
• New DiffViewerScreen modal with a scrollable RichLog widget rendering the patch using Syntax("diff", ...) highlighting
• render_diff_content() extracted from _render_patch_body() in widgets.py for reuse by both the inline transcript path and the modal
• DiffViewerScroll helper widget for scrollable diff content
  Dedicated Patch Storage
• Added last_diff: str | None field to TuiState
• Populated by record_tool_result() via the existing _result_patch() extractor — deterministic, no text scanning, provider-agnostic
• action_open_diff_viewer() reads directly from self.state.last_diff
• Removed fragile _extract_diff_from_tool_result() heuristic

Configurable Keybindings

• diff_viewer field added to TuiKeybindings with to_json() serialization
• _hidden_prompt_bindings() includes the diff_viewer candidate
• Default key swap reflected in /hotkeys help and /scoped-models description

Testing

• uv run pytest tests/test_tui_app.py tests/test_tui_config.py — 244 passed
• uv run ruff check src/tau_coding/tui/ — 0 errors
• uv run mypy src/tau_coding/tui/ — 0 errors