Adds `TELEGRAM_PROXY` environment variable as an override for the `proxy`
config key. Env var takes precedence over config.json so it can be set in
dotfiles/envrc without touching the stored session config.

Supported URL formats (same as `tgcli config set proxy`):
- socks5://host:port (with optional user:pass)
- http://host:port or https://host:port
- https://t.me/proxy?server=...&port=...&secret=... (Telegram share link)
- mtproto://host:port?secret=...

Closes #27

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NhaENbg5p8KWu8TBpHvHvc