Expand the common DKIM selector probe list

[sanmaxdev]

DKIM presence is probed at common selectors in COMMON_DKIM_SELECTORS in src/sentineldeck/scanners/email_security.py. DKIM absence is inconclusive, so more well-known selectors means fewer false "not detected" results.

What to do

• Add widely-used, static selectors for major providers, each with a short comment naming the provider (e.g. Yahoo sig1, Mailchimp k2/k3). Only add selectors that are stable and publicly documented; skip providers that use per-tenant dynamic selectors.

Acceptance

• Selectors added with a provider comment.
• pytest -q still green.

Comment to claim it.

[sage-s11]

I'd like to work on this. Kindly assign it to me.