docs: add Nylas banner to README

[nbarraclough]

Swaps the small centered logo for the full-width Nylas banner, matching the org profile branding (nylas/.github#4).

Nothing else changes — the SDK header, badges, nav links, product row, and Agent Accounts link are all already in place. This is the banner-only delta to bring the SDK READMEs in line with the new house style (wide banner + SDK header).

Banner asset is the same user-attachments image used on the org profile, so it renders identically.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (0c7c2a3) to head (7b03af0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #752   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files          43       43           
  Lines        1045     1045           
  Branches       92       92           
=======================================
  Hits         1032     1032           
  Misses         12       12           
  Partials        1        1           

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[tareeqa]

LGTM