feat(onboarding): add interactive walkthrough command

[hbrombeer]

What

grounds onboarding (aliases: onboard, quickstart) — a guided, explain-then-do walkthrough that takes a new developer from zero to a running app in five steps:

Login → Workspace (Bundle) → App anlegen (init) → push → spielen

Each step first explains the concept (German copy), then runs the real command after a Ja/Überspringen confirm. Skipping is always allowed; re-runnable any time.

How

• Reuses the existing subcommands (login / init / cluster up --bundle / push) via cobra Execute() — no duplicated logic.
• Bundle as the default workspace (per request): the Workspace step resolves the latest released bundle ref (ListBundleReleases → IsLatest, falls back to main) and provisions it. A partial/failed bundle is surfaced and offers "continue anyway" rather than dead-ending.
• Built on huh (already a dependency, previously unused); bails cleanly with a clear message when stdin is not a TTY.

Files: cmd/grounds/commands/onboarding/{onboarding.go,onboarding_test.go} + registration in main.go.

Verification

• go build ./... + go vet clean; package test green (metadata + non-TTY guard).
• grounds --help lists it; grounds onboarding --help renders; non-TTY input bails cleanly.
• ⚠️ The interactive flow itself was not run (no TTY in the build env) — needs a live grounds onboarding smoke test.

Notes / possible follow-ups

• Onboarding reliability depends on the Bundle pipeline's stability (it hard-defaults to a bundle).
• First-run nudge (suggest grounds onboarding after first login / empty workspace) — not included here.

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