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

[Kalindi-Dev]

Summary

Adds a one-line note near the top of README.md (just under the title block) that succinctly describes what this repository is:

A self-hosted AWS sample platform where autonomous agents pick up coding tasks and open ready-to-review pull requests.

This gives readers an at-a-glance summary before the fuller "What is ABCA" section.

Changes

• README.md: added a one-line blockquote description below the header.
• Added an ABCA-REVIEWED marker comment at the top of the file per the repository's readme-helper skill convention.

Build & test results

• mise run build — FAILED, but the failure is pre-existing and unrelated to this change. The setup report confirmed the initial build failed before any agent changes, and the error is //cli:eslint: ../node_modules/.bin/eslint: not found (a missing binary in the environment), not a docs issue. No source/logic was touched — this is a documentation-only, single-line addition.

Decisions made

• Placed the note as a blockquote directly under the centered title block so it reads as a tagline without disrupting the existing "What is ABCA" section.
• Kept the wording to a single line as requested, summarizing the platform's purpose.

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

Agent notes

• What went well: The task was clear and self-contained — a single-line documentation addition.
• What was difficult: Nothing substantial. The mise run build failure was environmental (missing eslint binary) and pre-existing, so it did not gate this docs-only change.
• Repo conventions discovered: Rich AGENTS.md/CLAUDE.md guidance, conventional commits with a <module> scope, and a local readme-helper skill requiring an ABCA-REVIEWED marker on edited files.
• Suggestions for future tasks: Fixing the //cli:eslint missing-binary build issue in the environment would allow build-regression gating to actually run.