docs(agent-compatibility): use copies for local install

[dajiaohuang]

Summary

• Replace the symlink-only local install example with a copy-based command.
• Explain that Cursor skips links whose targets resolve outside ~/.cursor/plugins/local, which is typical for a repository clone.

Related to #35.

Validation

• Confirmed the current Cursor plugin docs describe the external-symlink limitation.
• git diff --check passed.

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Note

Low Risk
Documentation-only change to local install instructions; no runtime or plugin behavior is modified.

Overview
Updates local install guidance in agent-compatibility/README.md so developers copy the plugin into ~/.cursor/plugins/local/ instead of symlinking from a clone.

The install example now uses cp -R rather than a symlink path, and a short note explains that Cursor ignores symlinks in that folder when the target lives outside the plugins directory—which is the usual case for a repo checkout—so copying is the right approach for local testing.

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