docs: align README, AGENTS.md and llms.txt with current SuperPlane positioning

[arunimshukla]

Summary

The repository describes SuperPlane as a DevOps control plane. superplane.com describes SuperPlane as an AI software factory for one-shot engineering. This PR aligns the repository text with the site.

Site copy today:

• Headline: "One-shot 30% of engineering tasks."
• Subheadline: "SuperPlane lets AI agents fully automate routine development work."

Repository copy today:

• README.md: "the open source DevOps control plane"
• AGENTS.md: "an open source DevOps control plane"
• scripts/generate-llms.mjs: "SuperPlane is an open source DevOps control plane for long-lived, event-driven workflows."

Changes

• README.md and AGENTS.md now describe SuperPlane as an open source AI software factory, and state the split of responsibility between agents and workflows.
• scripts/generate-llms.mjs updates the two hardcoded description strings that ship to /llms.txt and /llms-full.txt. Those two lines are the machine-readable summary that AI assistants read when someone asks what SuperPlane is, and today they answer "DevOps control plane".

Validation

npm run build completes and generates 94 pages, including the regenerated /llms.txt and /llms-full.txt.

Left unchanged on purpose

src/content/docs/concepts/glossary.md and src/content/docs/concepts/superplane-apps.mdx describe a SuperPlane app as "a control plane for long-lived, event-driven software engineering workflows". That reads as product terminology rather than positioning, so this PR does not touch it. Happy to align it in a follow-up if you want that.

Two things a PR cannot change

• The repository About description still reads "SuperPlane documentation". Suggested replacement: "Documentation for SuperPlane, the open source factory for one-shot engineering."
• The superplanehq/superplane README still opens with "an open source control plane for platform engineering". I can send the same alignment there if it is useful.