cli: Un-hide bootupctl backend and document its verbs

[cgwalters-bot]

bootupctl backend was hidden to keep the client side less confusing, but hidden commands cause more confusion than they avoid. Image build tooling uses it, and our own README, Dockerfile.bls and tests reference backend install, generate-update-metadata and set-default-bootloader, yet --help gives no hint that these exist or what they are for.

The first commit is a prep: bootupctl backend had its own hand-synced copy of the bootupd verbs, each individually hidden, so it now reuses DVerb and both entrypoints share one set of verbs and help text. The second shows backend in bootupctl --help, after the verbs for a booted system. It gets help text saying these commands act on an OS tree or target root at build/install time, and pointing users who want to update a running system at update and adopt-and-update. A unit test keeps backend and its verbs visible and documented.

Commands:
  status            Show components status
  update            Update all components
  adopt-and-update  Update all adoptable components
  validate          Validate system state
  backend           Low-level commands for building and installing OS images

For maintainers: this reverses the intent of the removed FIXME(lucab) comment, which planned to eventually drop backend from bootupctl after coreos/fedora-coreos-config#595. Since image build tooling still relies on it, documenting it seems better than keeping it half-hidden.

The issue also suggests detecting a booted system and giving clearer guidance there. That is not done here, because backend install is legitimately run on a booted host (e.g. bootc install to-existing-root), so a simple "systemd is running" check would misfire. The help text covers the guidance for now. So this is related to coreos#1119 rather than closing it.

Testing, on a 16-core devspace in rust:1.90.0 (the pinned lint toolchain in rust.yml): cargo fmt -- --check, cargo clippy --all-targets -- -D warnings, and cargo test --all-targets (40 passed, 1 ignored). I also checked the bootupctl --help and bootupctl backend --help output by hand.

Related: coreos#1119

Generated-by: https://github.com/cgwalters/#llms

---

Review draft in cgwalters-forge, not upstream yet. This section is removed when the PR is opened upstream.

• Upstream: coreos/bootupd, base main
• Board item: PVTI_lADOE9oHIs4BlJLczg9kEKI
• Fork CI: off; the devspace testing described above is this PR's CI, and upstream CI runs once it is opened there

To review:

• Approve, or comment /promote on a line of its own, to open it upstream, ready for review. Either covers only the commits pushed so far.
• If upstream requires DCO, approving also signs off: promote adds Signed-off-by: Colin Walters <walters@verbum.org> to the commits lacking it (the bot's and yours; anyone else's only if you ask), with you as committer.
• Add a /draft line (in the same comment or before) to open it upstream as a draft (/ready undoes that).
• Close to drop it.
• Edit the title and description freely: they become the upstream PR's. Review comments are squashed into the commits they concern, with a reply here.

[cgwalters-bot]

Not promoted: coreos/bootupd is human-text, so the title, body and commit messages sent there must be yours.

Next: edit this PR's title and body (dropping the Generated-by line), reword the commits and push them to this branch yourself, then comment /promote --human-text on a line of its own.

Generated-by: https://github.com/cgwalters/#llms

[cgwalters-bot]

Upstream LLM rules (advisory): coreos/bootupd's contribution policy record says human-text: code by the bot is fine, but the PR title and body, commit messages and comments must be cgwalters' own, except an answer on a thread where cgwalters summoned the bot. AI trailer: default (Generated-by: AI); nothing written, and precedent on main is Assisted-by (2 of the last 100 commits). The text sent there must be yours; the next comment says how.

Record: upstream-policy/coreos/bootupd.md, checked 2026-09-25 by the policy-check subagent (coordinator session 929c7a64, converted from an earlier read-only survey and re-read against current upstream).

Generated-by: https://github.com/cgwalters/#llms

[cgwalters-bot]

Upstream LLM rules (advisory): coreos/bootupd's contribution policy record says bot-ok: the bot may write the code and the text, with the AI trailer. AI trailer: default (Generated-by: AI); nothing written, and precedent on main is Assisted-by (2 of the last 100 commits). Nothing here holds this PR back: the bot promotes it now.

Record: upstream-policy/coreos/bootupd.md, checked 2026-09-25 by the policy-check subagent (coordinator session 929c7a64, converted from an earlier read-only survey and re-read against current upstream).

Generated-by: https://github.com/cgwalters/#llms

[cgwalters-bot]

Opened upstream as coreos#1169. Closing this review draft.