docs: put the AgentEnv Framework logo at the top of the README

[earakely-scale]

This brings the AgentEnv Framework logo over from the website repo (agent-env-website) and shows it at the top of the README.

What's added

File	What it is
assets/brand/lockup-light.png, lockup-dark.png	The mark with the wordmark, 1038×120. The README uses a <picture> element, so each reader sees the version that matches their theme.
assets/brand/icon.svg	The mark alone, as an SVG that follows the browser's dark mode. It's the website's favicon.
assets/brand/icon-512.png	The mark at 512px, for avatars and social cards.

The files are byte-identical to the website's. The sdist's only-include and the wheel's src/agent_env packaging don't pick up assets/, so neither package grows.

Checked

• The README tests that read its sections still pass: test_unread_keys, test_check_plugin_api and plugin_report_test (145 passed).

Not in this PR

• PyPI: pyproject.toml uses this README as the PyPI description, and PyPI doesn't resolve relative image paths or <picture>. The logo will show on GitHub but not on the PyPI page until the paths are absolute. That's easiest to switch once the repo is public.
• Explorer UI: its public/ icons and logos/agent-env-logo.svg still use the older logo. Rebranding the Explorer UI would be a separate change.

🤖 Generated with Claude Code

Confidence Score: 5/5

The PR appears safe to merge for its stated GitHub README goal.

Summary

The README now shows the AgentEnv Framework logo above its heading, with a light or dark lockup chosen to match the reader’s theme. The PR also adds standalone brand icons in SVG and PNG formats.

• Adds the theme-matched lockups to the README.
• Adds standalone icon artwork to assets/brand.

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