Add Windows operational helpers: port fallback, backup/restore, startup

[xodapi]

Why

Colleagues need fewer manual steps when port 3000 is busy, configs drift, or they want the proxy available after reboot.

Scope

• Graceful port fallback 3000 -> 3001 -> 3002.
• Update generated OpenCode provider config when fallback port is used.
• Backup/restore OpenCode config command.
• Optional Windows startup registration, disabled by default.

Acceptance criteria

• Fallback is explicit in console output and doctor checks.
• Backup files do not include secrets outside intended config scope.
• Startup registration can be listed and removed.