fix(cli): prompt for fresh OIDC login after logout

[grs]

Summary

Fix OIDC logout/login behavior so openshell gateway logout causes the next browser-based openshell gateway login to request a fresh identity-provider prompt. This lets users switch browser identities instead of silently reusing the previous SSO session.

Related Issue

Changes

• Added a per-gateway one-shot marker requesting prompt=login after OIDC logout.
• Cleared the marker after a successful token store or gateway registration removal.
• Updated the browser OIDC auth flow to append prompt=login when the marker is present.
• Updated CLI help text and gateway auth docs to describe the fresh-login behavior.
• Added unit coverage for marker lifecycle and authorization URL parameters.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)
• Manual testing

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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