fix(prompt): fail fast when stdin is not a terminal

[stantheman0128]

Description (required)

Every interactive prompt goes through pkg/prompt, which calls survey. When stdin is an open pipe that never sends input (CI step, agent shell, cron), survey draws the selector and blocks forever. On Windows it fails with the raw Incorrect function. instead.

This adds a stdin terminal check before each prompter method. Without a terminal it returns prompt.ErrNonInteractive:

ERROR	cannot prompt "Select a service": stdin is not a terminal; pass the required flags (see --help) or use -i=false

Selectors that auto-pick the only project or service never reach the prompter, so scripts that rely on that keep working. Piping answers into a prompt (printf 'y\n' | zeabur ...) already failed with EOF on main, so nothing that used to work is refused now.

I kept #279 out of this PR. It is a separate code path: on current main deployment log --deployment-id does not show the service prompt, but runtime logs then fail with serviceID is required for runtime logs because the command never looks up the deployment's service. I have a small fix for that and can open it separately.

Testing:

• New pkg/prompt/prompter_test.go swaps stdin for an open pipe. Before the fix, Select and MultiSelect hit the 3s timeout on Linux (WSL Ubuntu 24.04) and returned Incorrect function. on Windows. After the fix all six methods return ErrNonInteractive on both.
• Built Linux binaries of main and this branch, ran context clear and template get with stdin on an open FIFO and a dummy token: main hung until timeout (exit 124), this branch exits 1 right away with the message above.
• go test ./..., go build, golangci-lint run ./... (0 issues), gofumpt clean. I don't have make on this machine, so I ran the commands the Makefile wraps.

Written with help from Cursor (AI). I reviewed the change and ran the tests above.

Related issues & labels (optional)

• Closes Interactive selection prompts hang forever in non-TTY contexts instead of erroring #278
• Suggested label: bug

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