ci: merge the shared infra retries into a step's own retry rules

[antiguru]

set_retry_on_agent_lost gives every step four automatic retries for failures that belong to the infrastructure rather than the step: a lost agent, an agent stopped by the OS, exit 128 for a registry or GHCR hiccup, and exit 199 for a Rust ICE. It skipped any step that declared retry.automatic of its own:

retry = step.setdefault("retry", {})
if "automatic" in retry:
    continue

So a step asking for one extra rule lost all four rather than gaining a fifth. Nothing reports that, and the step then hard-fails on precisely the transient classes the shared rules exist for.

The loss is worth more than it looks. ci/plugins/mzcompose/hooks/command deliberately propagates 128 and 199 out of a job rather than collapsing them to 1, with a comment saying that overwriting them "turns a self-healing infra failure into a job that stays red until someone retries it by hand", and mzbuild.py exits 128 when a required image cannot be pulled. A step that overrode the rules went red on the first failed image pull where every other step retried twice — the MinIO Docker Hub outage fixed in #38802 is exactly that shape.

Merge instead of skip, adding only the rules a step does not already declare, and leave a step that retries on every exit status alone since it covers them already.

What regains coverage

Applying the old and new function to every checked-in template, thirteen steps were silently missing rules:

pipeline	steps	was missing
test	devel-docker-tags, console-sql-test	all four
test	check-merge-with-target	agent-lost only, having re-listed the other three by hand
nightly	devel-docker-tags, aws-real, aws-glue-schema-registry-real, 4× k8s-node-recovery-*, terraform-aws-upgrade, terraform-azure	all four
spec-sheet	devel-docker-tags	all four

After the change every step in every template carries all four, with no duplicated rules. The ci/deploy steps retry on exit_status: "*" and are left untouched.

Re-listing the shared rules inside a step stays correct and is now redundant rather than required; I left the existing re-listings in place rather than widen the diff.

Found by the post-merge QA review on #38797.

🤖 Generated with Claude Code

[bosconi]

👏 We should ask Claudex to find more of these.

[antiguru]

Thanks for the review!