[P3] Add --json to whoami and sessions list for scripting parity

[Bccorb]

Severity: P3 — Enhancement/Inconsistency

Where: src/commands/whoami.ts, src/commands/sessions.ts

Problem
config get/roles, users list/credentials, and org list/get/members list support --json, but whoami and sessions list — both natural scripting targets — do not.

Suggested fix
Add --json output to both for parity.

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