iso: use signed ostree transport for bootc switch at install time

[kylerankin]

Finding

[sec-check] #330: the ISO kickstart used

bootc switch --mutate-in-place --transport registry ghcr.io/projectbluefin/finpilot:stable

Plain registry (docker://) transport performs no sigstore/cosign verification and resolves the mutable :stable tag at install time. This is the only install path that skips signature checks, inconsistent with the repo's trust model (build-image.yml signs keyless via Fulcio; promote-main-to-stable.yml cosign-verifies before promotion; build/00-image-info.sh sets IMAGE_REF="ostree-image-signed:docker://...").

Fix

Switch to the signed OSTree transport, matching 00-image-info.sh:

bootc switch --mutate-in-place ostree-image-signed:docker://ghcr.io/projectbluefin/finpilot:stable

Dropping --transport registry, ostree-image-signed:docker:// wraps the registry transport with sigstore verification against Fulcio/Rekor, so the ISO install path verifies the same signed image as in-place upgrades.

— hive: backend=pi model=lemonade/Ornith-1.5-35B-A3B-GGUF-Q6_K

[kubestellar-hive]

This appears to duplicate #366, which makes the same change to iso/iso.toml.

Verified by reading both diffs in full: both replace bootc switch --mutate-in-place --transport registry ghcr.io/projectbluefin/finpilot:stable with bootc switch --mutate-in-place ostree-image-signed:docker://ghcr.io/projectbluefin/finpilot:stable, both for issue #330, and the claim is grounded — build/00-image-info.sh:41 already sets IMAGE_REF="ostree-image-signed:docker://...", so this aligns the ISO path with the existing trust model.

This PR is earlier, but #366 is a strict superset and I'd suggest it as the survivor:

• #366 adds %post --erroronfail. Without it, a failed bootc switch in kickstart %post is swallowed and the install completes on the unswitched base image — with the signed transport now able to reject an image, a verification failure should fail the install rather than pass silently.
• #366 also updates the FORK NOTE comment to the ostree-image-signed:docker:// form; this PR leaves that comment telling forks to use the old unsigned ref, and instead appends a long inline # annotation on the command line itself.

Flagging for a maintainer: if you agree, this one could be closed in favour of #366. I have not closed anything.

— hive: agent=reviewer backend=copilot model=claude-fable-5 copilot=1.0.78