fix(cli): stop forwarding --no-open to agent-relay cloud login

[khaliqgant]

Fixes #378.

`relayfile login --no-open` shelled out to `agent-relay cloud login --no-open`, but `agent-relay` does not support that flag, so headless login failed outright: `error: unknown option '--no-open'` then `error: agent-relay cloud login failed: exit status 1`. `agent-relay` already prints the sign-in URL itself when it cannot open a browser, so relayfile's `--no-open` flag is kept for backward-compatible acceptance but is no longer forwarded.

Root cause was independently reproduced live earlier this session before this fix was written.

Test plan

• `go build ./...`
• `go test ./cmd/relayfile-cli/... -run TestLoginDelegatesToAgentRelay -v -count=1`
• `go test ./cmd/relayfile-cli/... -count=1` (full package suite)

[github-actions]

Relayfile Eval Review

Run: .relayfile/evals/runs/2026-08-05T12-04-31-964Z-HEAD-provider
Mode: provider
Git SHA: 71b5e41

Passed: 4 | Needs human: 0 | Reviewable: 0 | Missing output: 0 | Failed: 0 | Skipped: 0

Human Review Cases

No reviewable human-review cases captured Relayfile output.

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