docs: lead the README with the loop, not a third wordmark

The use case now lands in the first screen. A new assets/flow.svg draws
the whole loop in one picture: push a markdown file to a draft, publish
or schedule it, update a live post through the audit gate, and pull the
whole archive back down. That diagram replaces the banner in the hero,
because the logo, the H1 and the banner were three copies of the
wordmark before the reader learned anything.

Quickstart moves above the essay. Install and first push used to sit at
line 100, under 900 words on why Substack has no API. That essay is
still here, further down, and it gained a table contrasting the web
editor with this tool.

Four GitHub alert callouts replace flat prose for the cookie, the audit
gate, Note scheduling and the destructive update. The twelve features
become a card grid instead of a flat table. The 22-command reference and
the docs list collapse into details blocks, which cut the rendered page
from 12,506px to 9,851px.

Wires in setup2, push2 and agent2, which were generated but never
referenced, and adds covers.svg with its session source for a later
pass. agent.svg, push.svg, quickstart.svg and banner.svg are now
unreferenced but kept, since banner.png remains the social preview.

Verified through GitHub's own markdown API: all four alerts fire, both
details blocks open, and all nine image paths resolve. Prose is clean on
the slop scorer and over-long sentences dropped from 19 to 10.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: HighnessAtharva