Add JSON output to `register` command (12.13)

[josecelano]

Summary

Add --output-format json support to the register command, as part of roadmap section 12 (Epic #348).

Roadmap task: 12.13
Parent Epic: #348

Specification

See docs/issues/396-add-json-output-to-register-command.md for the full spec (to be committed after issue creation).

JSON Output Format

{
  "environment_name": "my-env",
  "instance_ip": "192.168.1.100",
  "ssh_port": 22,
  "registered": true
}

Field	Type	Description
environment_name	string	Name of the environment the instance was registered to
instance_ip	string	IP address of the registered instance
ssh_port	integer	SSH port used for the instance (default 22)
registered	boolean	Always true when the command exits successfully

Implementation Notes

• RegisterCommandController::execute() returns Environment<Provisioned> — the DTO is built from this rich result
• DTO: RegisterDetailsData { environment_name, instance_ip, ssh_port, registered }
• Named constructor: from_environment(env: &Environment<Provisioned>)
• Strategy Pattern — same as all prior commands in this epic
• complete_workflow() needs updating to receive &Environment<Provisioned> alongside output_format

Acceptance Criteria

• cargo run -- register my-env 192.168.1.100 --output-format json 2>/dev/null produces valid JSON
• Text mode output is unchanged
• All existing tests pass
• cargo run --bin linter all passes