checkpatch: add support for Assisted-by tag

checkpatch: add support for Assisted-by tag

The Assisted-by tag was introduced in
Documentation/process/coding-assistants.rst for attributing AI tool
contributions to kernel patches.  However, checkpatch.pl did not recognize
this tag, causing two issues:

  WARNING: Non-standard signature: Assisted-by:
  ERROR: Unrecognized email address: 'AGENT_NAME:MODEL_VERSION'

Fix this by:
1. Adding Assisted-by to the recognized $signature_tags list
2. Skipping email validation for Assisted-by lines since they use the
   AGENT_NAME:MODEL_VERSION format instead of an email address
3. Warning when the Assisted-by value doesn't match the expected format

Link: https://lkml.kernel.org/r/20260311215818.518930-1-sashal@kernel.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
Reported-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>