Add guided Relayfile cloud setup

[khaliqgant]

Summary

• make bare relayfile and relayfile setup run the guided Cloud onboarding flow
• add browser Cloud login, workspace create/join, Nango connect session polling, credential/catalog persistence, and handoff to the existing mount loop
• document the new first-run flow and add focused CLI coverage

Tests

• go test ./...

Note: unrelated local trajectory index/untracked trajectory files were left out of this PR.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Use login callback API URL for subsequent Cloud requests

runCloudLogin parses and returns api_url from the callback (via readCloudCredentialsFromQuery), but setup still constructs cloudClient with the original cloudAPI value. If Cloud login redirects through a front door and returns a different API origin (regional/tenant-specific), workspace creation/join requests are sent to the wrong host and setup fails after successful auth. Use tokenSet.APIURL (or overwrite cloudAPI after login) before creating the API client.

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