feat: add --environment flag to sqlmesh janitor for scoped cleanup

[mday-io]

Description

Add an --environment / -e flag to sqlmesh janitor that scopes cleanup to a single named environment, and fix sqlmesh invalidate <env> --sync to only clean up the named environment rather than all currently-expired environments.

New flag: sqlmesh janitor --environment <name> removes views, schemas, and state records for the named environment only. If the environment is not expired or does not exist, a warning is emitted. Global snapshot expiration and interval compaction are skipped when this flag is set — those operations are cross-environment and cannot be safely scoped to a single one.

Bug fix: sqlmesh invalidate <env> --sync previously called _cleanup_environments() with no filter after marking the target environment expired, which caused all other currently-expired environments to be deleted as a side effect. It now scopes the cleanup to the named environment only.

Test Plan

• Tested manually against the sushi example project using --gateway duckdb_persistent with two environments (dev1, dev2). Invalidating dev1 and running sqlmesh janitor --environment dev1 cleaned up dev1 while leaving dev2 untouched.
• Verified the warning message fires when targeting an environment that is not expired or does not exist.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO