Add /plan slash command for plan-only proposals in the side panel

/plan [goal] runs a one-shot, plan-only turn: the agent inspects the
repo, writes a structured plan (Goal, Scope, Approach, Validation, Open
questions) to the side panel, and stops without editing files. It only
turns the plan into a todo list and starts work once the user approves.

Wired through local and remote dispatch, the command registry,
arg-completion, /help <plan> detail, and the help overlay. Includes unit
tests for the parser/prompt and app-level dispatch tests.