No harm in describing it in the README, esp. as other development
programs are to be added imminently.

…hored input YAML

This allows us to author the workflow using YAML anchors/references
(especially with merge keys) since GitHub Actions doesn't otherwise
support those YAML features.

The lack of support is a shame because GitHub Actions workflows can be
very repetitive and anchors/references/merges are a decent solution to
that.  I'm about to add substantial conceptual replication that we won't
want to maintain concretely replicated in the file.

Since the generated file must be checked in, a new CI step ensures the
generated file matches the authored file and the generated file is
excluded from git diffs by default.  An optional local pre-commit hook
is also available for making sure you craft commits that won't run afoul
of the CI check later.  Or you can, as necessary, run `make` manually
before committing.

This lets us more easily test it in development.

This avoids the disconnect between the success/failure status of a
GitHub Actions workflow run and the actual AWS Batch job, which makes
for easier reporting and debugging and generally less cognitive
dissonance.

GitHub Actions workflow _runs_ have a very high max timeout of 35 days,
but each _job_ in a workflow has a much lower max timeout of 6 hours.
Many of our builds should be less than 6 hours¹, but here we support
builds up to 24 hours by chaining together 4 GitHub Actions jobs.  We
can add more jobs to the chain if we need to, but I don't foresee that.

Nearly all of our builds are in public repos, which means they won't
consume usage minutes from our GitHub Actions quota.  However, they
_will_ consume concurrency limits from the quota.  We were already
frequently bumping into the default free-tier quota of 20 concurrent
jobs, so adding more long-running jobs (to wait around for the AWS Batch
jobs) was a nonstarter until we upgraded to a Team plan with its
corresponding quota of 60 concurrent jobs.²

¹ As of 24 Jan, all Batch jobs except one in the prior week were sub 12
  hours.  The exception was the GISAID ncov-ingest job launched on 16 Jan.
  Next longest jobs were 10.5 hours, all GenBank ncov-ingest jobs.
  <nextstrain/private#95 (comment)>

² <https://bedfordlab.slack.com/archives/C7SDVPBLZ/p1705024616778389>
  <https://github.com/tsibley/blab-standup/blob/HEAD/2023-08-22.md>