Add CLAUDE.md that imports AGENTS.md via @-syntax

[aiolibsbot]

What do these changes do?

Add a CLAUDE.md at the repository root containing a single
@AGENTS.md line. Claude Code resolves the @-prefix as an
import, so the project's LLM contributor rules in AGENTS.md
load automatically when an agent starts a session in this repo,
without duplicating any content.

Mirrors aio-libs/yarl#1696.

Are there changes in behavior for the user?

No.

Is it a substantial burden for the maintainers to support this?

No. The file is one line and points at the existing AGENTS.md,
so future edits stay in AGENTS.md only.

Related issue number

N/A

Checklist

• I think the code is well written
• Unit tests for the changes exist — N/A, docs-only
• Documentation reflects the changes
• If you provide code modification, please add yourself to CONTRIBUTORS.txt — N/A, docs-only
• Add a new news fragment into the CHANGES/ folder

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Quality Report

Changes: 2 files changed, 6 insertions(+)

Code scan: clean

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codspeed-hq]

Merging this PR will improve performance by 17.26%

⚡ 1 improved benchmark
✅ 244 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_multidict_insert_str[ci-c]	99.5 µs	84.8 µs	+17.26%

Tip

Curious why this is faster? Comment @codspeedbot explain why this is faster on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing aiolibsbot:koan/claudemd-import-agents (57c5c98) with master (fc6933d)}