Move CI to ubuntu-26.04; build COPY-heredoc README sample unconditionally

[Andrei-Errapart]

Background

PR #4 treated the COPY <<EOF here-document failure under Podman as an engine
limitation and made tests/0029 skip the installer sample under Podman.
Upgrading a dev box to Debian 13 (trixie) disproved that — it's only a
Buildah-version gap:

Builder	COPY <<EOF
Docker / BuildKit	✅ always
Buildah 1.28.2 (Debian 12) / 1.33 (Podman 4.9.3, Ubuntu 24.04)	❌
Buildah ≥ 1.39 (Podman 5.x; Debian 13 trixie, ubuntu-26.04)	✅

Change

• Drop the engine probe/skip code; build the installer sample
  unconditionally (engine-aware cleanup retained).
• Both CI jobs → ubuntu-26.04 (Podman 5.7 / Buildah 1.42), so the heredoc
  sample builds under Podman too.
• DNS workaround for the Podman job. On ubuntu-26.04 the host resolver is
  the systemd-resolved stub (127.0.0.53), which isn't reachable from inside a
  Podman build container's netns — so apt-get/apk in a podman build fail
  with Temporary failure resolving … (24.04 / Podman 4.9 didn't hit this). The
  job writes a containers.conf.d drop-in pointing build containers at public
  resolvers (1.1.1.1 / 8.8.8.8). run-dockerfile never injects --dns; this is
  a runner-environment quirk.
• README keeps an engine note for users on an older Podman/Buildah (use a
  sibling-file COPY); CLAUDE.md documents the Buildah-version gap and the DNS
  drop-in.

Verification

• Locally (trixie / Podman 5.4.2): 0029 builds the heredoc sample under Docker
  and rootful Podman; the dns_servers drop-in propagates into podman build
  containers (/etc/resolv.conf → 1.1.1.1).
• CI (this PR) runs the whole suite on ubuntu-26.04 — the authoritative check for
  the runner move + DNS fix, including the heredoc sample under Podman 5.7.

Supersedes the probe approach; follow-up to #4.