Add AGENTS.md

Signed-off-by: Jai Luthra <me@jailuthra.in>

AGENTS.md

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+# AGENTS.md
+
+This repo provides NVENC/NvFBC patches for Nvidia drivers on Linux (`patch.sh`, `patch-fbc.sh`)
+and Windows (`.1337` files under `win/`, applied via Win_1337_Apply_Patch).
+
+## Golden rules
+
+- **Never hand-edit generated artifacts.** `README.md`, `win/README.md`, and `drivers.json`
+  are generated. `drivers.json` is the single source of truth; the READMEs are rebuilt from it
+  and the `tools/readme-autogen/templates/` templates.
+- **Never hand-edit the `patch_list` in `patch.sh`/`patch-fbc.sh`.** Use the autopatch scripts.
+- The canonical end-to-end flow is `.github/workflows/gen_linux_patches.yml` and
+  `.github/workflows/gen_win_patches.yml` — mirror them when working manually.
+
+## Testing: >12 simultaneous NVENC sessions
+
+The unpatched driver fails past 12 concurrent sessions; the patch must allow 13+.
+
+- **Linux**: `tools/patch-tester/patch-tester.sh` — runs 13 concurrent `h264_nvenc` encodes
+  (1M–13M bitrate) against `testsrc` via CUDA. All 13 must complete without error.
+  Requires ffmpeg built with NVENC/CUDA support and the patched driver installed.
+- **Windows**: `win/tools/patch-tester/patch-tester.ps1` (or `.bat`) — same 13-stream test.
+
+Test against the patched driver and confirm 13 sessions succeed on a consumer GPU before opening a PR.