integration: move local dashboard secrets out of plaintext inventory JSON

[ethayu]

Problem

The integration dashboard currently stores GitHub tokens and Pi SSH passwords in the local inventory file (src/integration/.integration_inventory.json by default). This was accepted for v1 competition ops, but it is not a good long-term security posture.

Current behavior

• OperatorConfig persists github_token in the local inventory JSON
• TargetRecord persists ssh_pass in the same local inventory JSON
• The new hardware deploy flow depends on that inventory for dashboard target management

Follow-up work

• Move secret storage to the desktop OS keychain / secret store
• Keep non-secret target metadata in the inventory JSON
• Preserve export/import for non-secret fields while handling secrets separately
• Define migration behavior from plaintext inventory files

Notes

• For the current production slice, plaintext local storage remains allowed for competition ops
• The code now includes an explicit comment marking this as a temporary v1 choice