docs(readme): add one-line description of the repo

[Kalindi-Dev]

Summary

Adds a one-line note near the top of README.md describing what this repository is, giving readers an at-a-glance summary before the fuller "What is ABCA" section.

The line added (as a blockquote just under the header block):

A self-hosted AWS platform where autonomous agents pick up coding tasks, work in isolated cloud environments, and open ready-to-review pull requests.

Build and test results

• mise run build — FAILED, but the failure is a pre-existing environment issue unrelated to this change: [//cli:eslint] sh: 1: ../node_modules/.bin/eslint: not found. The setup notes confirm the initial build already failed before any agent changes. This PR only touches README.md (Markdown), so it cannot affect the build/lint outcome.

Decisions made

• Placed the note as a Markdown blockquote immediately after the centered header block and before ## What is ABCA, so it reads as a concise tagline without duplicating the existing detailed description.
• Per the active repo skill, added an <!-- ABCA-REVIEWED --> marker comment near the top of the edited file.

Agent notes

• What went well: The change is small and self-contained; the README structure was easy to understand.
• What was difficult: The build pipeline fails in this environment due to a missing eslint binary path under cli/, unrelated to the change. Build-regression gating is noted as inert for this repo.
• Conventions discovered: Rich AGENTS.md/CLAUDE.md guidance, conventional commits with module scope, and Starlight docs mirroring for docs/ prose (not relevant to a root README edit).
• Suggestions: Fix the cli:eslint task's binary path (../node_modules/.bin/eslint not found) so builds pass in fresh environments.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of the project license.