Feature: Add tui_help tool for on-demand command reference

[wangdayong228]

Problem

DeepSeek TUI's system prompt includes Mode/Approval-Policy labels but no reference to its own built-in slash-commands (/mode, /config, /approval, /compact, etc.).

When a user asks "how do I switch modes?", the agent must either:

1. Reason from incomplete prompt fragments → hallucinates
2. Run deepseek --help ad-hoc → wastes time and often doesn't occur to the agent

Proposal: tui_help — an on-demand retrieval tool

Add a tool following the diagnostics pattern:

tui_help(query: "mode" | "approval" | "compact" | ...)

• tui_help("mode") → returns commands for switching Plan/Agent/YOLO
• tui_help("approval") → returns approval-policy commands
• tui_help("") → returns a cheatsheet of common slash-commands
• Implementation reads from the CLI binary, single source of truth

Why on-demand > static prompt injection

Aspect	Static injection	tui_help
Token cost	Wasted every session	Only when queried
Precision	Full help, many irrelevant commands	Scoped to query
Freshness	Two copies drift	Live from binary

Fits existing pattern

diagnostics and doctor already exist for self-inspection. tui_help extends this cleanly.

deepseek-tui 0.8.37 (4c32a31)