Add topic: ai-agent

[TorstenC]

Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/main/CONTRIBUTING.md.
• This change is not self-promotion.

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection
• Something that does not neatly fit into the binary options above

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Curating a new topic or collection

• I've formatted my changes as a new folder directory, named for the topic or collection as it appears in the URL on GitHub (e.g. https://github.com/topics/[NAME] or https://github.com/collections/[NAME])
• My folder contains a *.png image (if applicable) and index.md
• All required fields in my index.md conform to the Style Guide and API docs: https://github.com/github/explore/tree/main/docs

The topic ai-agent is used by ~8,900 repositories (plus ~20,900 under the plural alias ai-agents), yet has no curated description in GitHub Explore. AI agents — autonomous software systems that use foundation models to reason, plan, and act — are one of the fastest-growing categories on GitHub, spanning coding assistants, browser automation, multi-agent teams, and execution sandboxes. A curated topic page would help developers discover and contextualize this ecosystem.

No image included — no universally recognized logo exists for this general concept.

[Copilot]

Pull request overview

Adds a curated GitHub Explore topic page intended for the ai-agent topic.

Changes:

• Introduces a new topics/ai-agent/index.md topic definition and long-form description.

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