[P3] Guard unknown commands and render non-Error throws safely

[Bccorb]

Severity: P3 — Improvement/UX

Where: src/index.ts:114, 117-119; src/commands/whoami.ts:19; src/commands/sessions.ts:30

Problem
Any unmatched first arg falls through to runCLI (init), so seamless verfy (typo) silently scaffolds a directory with no feedback. The top-level catch prints Error: undefined when a non-Error is thrown; (err as Error).message has the same issue in whoami/sessions.

Suggested fix
Keep the seamless <name> shortcut but reject flag-like/known-typo first args with a "did you mean" hint; render thrown values safely (String(err) fallback).

Filed from an internal code audit (2026-07). File references are against main at audit time.