scripts/checkpatch: add Assisted-by: tag validation

scripts/checkpatch: add Assisted-by: tag validation

The coding-assistants.rst documentation defines the Assisted-by: tag
format for AI-assisted contributions as:

  Assisted-by: AGENT_NAME:MODEL_VERSION [TOOL1] [TOOL2]
This format does not use an email address, so checkpatch currently
reports a false positive about an invalid email when encountering this
tag.

Add Assisted-by: to the recognized signature tags and standard signature
list. When an Assisted-by: tag is found, validate it instead of checking
for an email address.

Examples of passing tags:
- Claude:claude-3-opus coccinelle sparse
- FOO:BAR.baz
- Copilot Github:claude-3-opus
- GitHub Copilot:Claude Opus 4.6
- My Cool Agent:v1.2.3 coccinelle sparse

Examples of tags triggering the new warning:
- Claude coccinelle sparse
- JustAName
- :missing-agent

Cc: Jani Nikula <jani.nikula@linux.intel.com>
Assisted-by: Claude:claude-opus-4.6
Co-developed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260327154157.162962-1-harry.wentland@amd.com>