[air] Move AIR commands out of experimental

[caroline-db]

What did you change, and why?

• Move AIR commands and acceptance tests from their experimental locations into cmd/air and acceptance/air.
• Register databricks air as a top-level command and update help, ownership, and release notes.
• Keep a hidden databricks experimental air command that returns an error directing users to databricks air.
• Note that AIR commands remain under active development.
• This PR does not otherwise change AIR APIs, configuration formats, or workload behavior.

How do you know it works?

• Ran go test ./cmd/air ./cmd/experimental -count=1.
• Ran all AIR and root-help acceptance tests.
• Ran repository checks and Go lint with zero issues.

This PR was written with Codex.

[eng-dev-ecosystem-bot]

Integration test report

Commit: 0b847b3

Run: 35682194235

	Env	✅​pass	🙈​skip	Time
✅​	aws linux	276	32	4:17
✅​	aws windows	278	30	3:19
✅​	azure linux	275	32	4:04
✅​	azure windows	277	30	3:19
✅​	gcp linux	276	32	4:31
✅​	gcp windows	278	30	4:41

Top 3 slowest tests (at least 2 minutes):

duration	env	testname
4:40	gcp windows	TestAccept
3:18	aws windows	TestAccept
3:17	azure windows	TestAccept