refactor ZED capture into modular camera class with env config

Extract camera lifecycle (open, warmup, grab, close) and
environment-based configuration into src/zed/camera.{h,cc}.
Thin main.cc now only handles the dora event loop.

Closes #2

GEMINI.md

@@ -0,0 +1,3 @@
+# GEMINI.md
+
+See [AGENTS.md](AGENTS.md) for repository guidance.