[lib] expand agent detection coverage

[davidmokos]

Summary

• Expands environment detection with the AI_AGENT override, normalized aliases, and additional Codex, Cursor, Claude, OpenCode, Augment, and Copilot signals.
• Adds Claude Cowork, Augment CLI, and v0 identities plus a reusable filesystem strategy for Devin's local marker.
• Keeps Cursor variants on the existing stable ID, rejects empty signals and generic REPL_ID false positives, and documents and tests the new behavior.

Validation

• npm test