Set up dev environment (Linux) + fix CMake configure break

[CommanderPho]

Summary

Sets up the development environment for LabRecorder (C++/Qt6 LSL recorder) on Linux and verifies it builds and runs end-to-end.

Two code changes were required:

• CMakeLists.txt — Fixed a malformed if(APPLE) block (a bad merge left an unterminated add_executable(LabRecorderCLI ...) with a duplicate nested add_executable). This caused Parse error. Function missing ending ")" on every platform, blocking all configures/builds. Removed the stray duplicate.
• AGENTS.md — Added with Cursor Cloud specific build/run instructions and non-obvious caveats (default c++ is clang → needs libstdc++-14-dev; committed LabRecorder.cfg has a non-writable Windows save path; how to run an LSL sender + record via CLI/GUI).

Environment

• System deps: build-essential, cmake, qt6-base-dev, freeglut3-dev, libpugixml-dev, libstdc++-14-dev, plus liblsl (1.16.2 noble .deb, matching CI).
• Build: cmake --preset linux-gcc-release + cmake --build --preset linux-release -j.

Verification

• ✅ Configure + build (LabRecorder, LabRecorderCLI, xdfwriter, testxdfwriter).
• ✅ testxdfwriter runs (exit 0, writes test.xdf).
• ✅ CLI end-to-end: started a custom liblsl EEG sender, recorded with LabRecorderCLI out.xdf 'type="EEG"' → valid XDF: file containing TestEEG/EEG.
• ✅ GUI end-to-end: launched LabRecorder, discovered TestEEG via Update, set a writable save path, Start → record → Stop → 70 KB valid XDF.

labrecorder_gui_recording_demo.mp4

GUI recording an LSL stream to XDF (status bar shows growing file size, then Stopped):

GUI recording in progress
GUI recording stopped

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