fix: make Codex recall compressor tests cross-platform

[luojiyin1987]

Summary

• Replace the POSIX shell fake Codex command with a Node.js script.
• Add the npm entry point used by the Windows launch resolver.
• Build PATH with the platform delimiter.
• Detect Codex compressor spawns by the exec argument.

Root cause

The test helper created an extensionless shell script.

It also joined PATH entries with a colon.

Native Windows could not resolve this fake command.

The tests could call a real Codex installation and exercise the fallback path.

Impact

The compressor tests now use the fake launcher on Windows, macOS, and Linux.

The tests no longer depend on a user-installed Codex command.

Validation

• node --test --test-name-pattern "relevance compressor|bounded deterministic" examples/codex-memory-plugin/scripts/auto-recall.test.mjs
• node --test examples/codex-memory-plugin/scripts/auto-recall.test.mjs
• Codex memory plugin test set: 66 tests passed.

Closes #3955