Multi-account support

[andreahlert]

Description

Support managing multiple Substack accounts from a single installation.

Features

• Named profiles stored in config
• Switch active account via CLI flag or env var
• TUI account switcher
• Per-account automation rules

CLI

postcli-substack auth login --profile work
postcli-substack auth login --profile personal
postcli-substack posts list --profile work
postcli-substack auth switch personal

Storage

Extend ~/.config/postcli/ to support multiple .env.<profile> files or a unified config file.