feat(cluster): confirm before bootstrap turns the machine into a cluster node

[incognick]

burrow cluster bootstrap installs k3s and burrowd on the current machine, so an accidental run on a laptop (e.g. curl -sfL <script> | sudo sh) would convert it into a cluster node. It now warns and requires confirmation before any destructive action.

What changed

• A pre-flight warning is printed before bootstrap mutates the machine: it states that bootstrap installs k3s and turns THIS machine into a Burrow cluster node (a systemd k3s service, burrowd, and Postgres), and to run it on a fresh server or VPS, not a laptop.
• The confirmation ("Continue? [y/N]") defaults to No: an empty line or anything but y/yes declines and aborts cleanly ("Aborted; nothing was changed.") before any install.
• The answer is read from /dev/tty, not stdin, so the prompt still reaches the user under curl -sfL <script> | sh, where the process stdin is the piped install script rather than the keyboard. The read sits behind a confirmFn seam (like the existing newIPDetector/newK3sInstaller seams) so tests inject the answer without a real terminal.
• A --yes bool flag skips the prompt for intentional automation. The get.burrow.dev script does not pass it, so an interactive user always gets the prompt.
• When there is no controlling terminal (/dev/tty cannot be opened) and --yes was not passed, bootstrap refuses with guidance to pass --yes rather than hanging or silently proceeding.
• The already-installed idempotent-skip case stays a no-op: when k3s is already running there is nothing destructive to do, so no confirmation is prompted.

Tests

Added to cmd/burrow/bootstrap_test.go, all seams faked (no real network, k3s, or cluster): --yes skips the prompt and installs; a confirmed "yes" proceeds; a declined "no" aborts cleanly with the installer never called; no-tty without --yes errors with --yes guidance and does not install; an already-running box skips the prompt. The existing bootstrap tests stay green.

https://claude.ai/code/session_012euqxgVqHP5yQWi2HxDPKt