feat: add --email and --ssh-key flags to signup command

[AlephNotation]

Summary

Adds --email and --ssh-key flags to vers signup so users and agents can specify credentials explicitly instead of relying on auto-detection.

Usage

# Fully explicit (no auto-detection, no stdin)
vers signup --email me@co.com --ssh-key ~/.ssh/id_ed25519.pub --org myorg

# Just override email, auto-detect SSH key
vers signup --email me@co.com

# Just override SSH key, read email from git config
vers signup --ssh-key /path/to/key.pub

Validation

• Email: RFC 5322 format via net/mail.ParseAddress
• SSH key file:
  • File exists and is not a directory
  • Not too large (16KB limit — catches accidental private key files)
  • Valid key type prefix (ssh-ed25519, ssh-rsa, ecdsa-sha2-*, sk-*)
  • Has base64 key data field

Tests

• internal/auth/shellauth_test.go — 12 test cases for SSH key validation (valid keys, missing files, empty, wrong type, oversized, private key detection)
• cmd/signup_test.go — flag existence and defaults

[AlephNotation]

obsoleted by #172