feat: add cmsend-notify to alert chatmail groups about CI failures/recoveries

[j4n]

This is a demonstration to use cmsend to notify on CI run state changes (e.g., pass -> fail and vice versa). It is motivated by an existing nightly CI job which provoked the question "how do we get notified on failure?". In my understanding, this would be possible either through:

• A healthchecks.io-style check that ping on success
• A notification sent on status change, either by email, for example, to a team-bot instance, or by
• Notifying a chatmail group directly without going over legacy email.

Here, we pursue the third option, leveraging the existing work of cmsend to implement a cmsend-notify.yml workflow that sends updates to a chatmail group. The initial setup of the profile with group join is done by hand, then the state is compressed with a helper, scripts/pack-cmsend-profile.sh helper to pack/unpack cmsend profiles into Action secrets. Once live, the state lives in an encrypted cache artifact, and is only updated by a .github/workflows/cmsend-sync.yml nightly helper workflow keeps the profile alive and in sync.

TODO: drop temporary pins to this and the cmping feature branch

This relies on the --name and --fetch-only flags of chatmail/cmsend#6

[hpk42]

repeating the changed file list in the description that i also get with "changes" tab does not add value.
Please edit your description and say what you want to achieve, and what needs it, in your own words. Based on that i can read the diff, so you don't need to preview or summarize the changes, rather motivate and reason about them.

Also note that i commented on the cmsend-branch you reference chatmail/cmsend#6 (comment)

I'd like to have non-generic, concrete use cases for "cmsend" before proliferating it here.

[j4n]

I edited the body text.