[feature] Epic: Add CLI command for trusted-client sessions

[marwannettour]

Summary

Add command-line support to create automatic cloud sessions with already trusted clients.

The target user experience is a command such as:

bytesync session-start <clientId1>,<clientId2>,<clientId3>

The command should create a session including the current client and invite the selected trusted clients through the automatic trusted-device flow.

Context

ByteSync already has command-line and headless concepts such as --join, --inventory, --synchronize, and --no-gui. This epic turns trusted-client automation into an operator-friendly workflow.

Proposed scope

• Add a command or argument set for starting an automatic trusted session.
• Accept one or more target trusted client IDs.
• Validate client IDs before starting session creation.
• Return clear exit codes and messages for scripting.
• Support headless usage with --no-gui.
• Provide optional machine-readable output for created session information and member statuses.
• Surface actionable errors when clients are missing, offline, not mutually trusted, not granted, or rejected by challenge validation.

Out of scope

• Full synchronization scripting DSL.
• Automatic enrollment of new clients.
• UI changes.

Acceptance criteria

• A CLI command can start a cloud session with one or more already trusted clients.
• The command includes the local client automatically.
• The command validates the maximum remote client limit.
• The command fails safely when any selected client cannot be auto-joined.
• Exit codes are documented for success, validation failure, trust failure, grant failure, network failure, and timeout.
• The command works in --no-gui mode.
• Tests cover argument parsing, validation, success, and main failure paths.

Dependencies

• Requires the automatic session domain flow.
• Requires trusted-device grant validation.
• Requires signed challenge-response for automatic joins.