Extends the --agent flag with a more general --env flag that allows
setting arbitrary environment variables when starting a witness.
Precedence (highest to lowest):
1. CLI --env overrides
2. Role bead env_vars
3. config.AgentEnv() defaults
Examples:
gt witness start greenplace --env ANTHROPIC_MODEL=claude-3-haiku
gt witness restart greenplace --env DEBUG=1 --env VERBOSE=true
Co-authored-by: joshuavial <git@codewithjv.com>