docs: sync the action logo with the new org branding

The comment header and the two README previews embed assets/logo.png at
width="20". That file was still the old wordmark — a 128x128 badge with
"COMMIT" set across it — while the org rebranded to a wordless mark in
commit-check/.github#49.

At the size it is actually displayed the wordmark is unreadable, which is
the reason the new avatar dropped the text in the first place; the
branding README says so directly: "GitHub shows org avatars at around
40px in most places, and at that size a wordmark is an unreadable smudge."
The header renders at half that again.

Re-rendered from branding/logo-mark.png, the asset that README nominates
for "the mark alone, transparent, where a background already exists" —
which is exactly the case here, since the mark sits on the comment's own
background. Downscaled 1024 -> 128 with Lanczos, keeping the file's
existing dimensions so nothing that references it has to change.

Measured on the result: RGBA preserved (alpha extrema 0-255, so the
transparent ground survived rather than being flattened to white), and
the dominant opaque colour is (44, 156, 205) = #2c9ccd, the brand colour
the branding README documents. 4994 -> 3001 bytes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U9zFxq8V4qxG4aMzJhGBFn

Author: shenxianpeng