docs: add coverage command to AGENTS.md

[sonesuke]

Walkthrough: Add Coverage Command to AGENTS.md

This PR updates AGENTS.md to include instructions for the newly added test coverage command.

Changes Made

Documentation Update

• Added mise run coverage to the "Tools" section in AGENTS.md.
• This ensures that AI agents working on the project are aware of the available command for generating test coverage reports via cargo-llvm-cov.

Verification Results

Build Check

• Verified that the changes do not affect the build or tests.
• Formatting checked with cargo fmt.