docs: replace README hero with new ComputerAgent render

New hero showing the retro Compaq V750 + the one-framework code line
(cagent = ComputerAgent(OpenGap, harness=, runtime=, memory=)) wiring
to the five orthogonal ports — source, harness, runtime, memory, comm —
with real provider logos (GitHub/GitLab/Bitbucket, E2B/OpenShell,
Cognis/mem0/supermemory, WhatsApp/Slack/Teams).

Author: shreyas-lyzr