feat: Add mod build environment with version + git hash

Add `env:mod` PlatformIO environment that sets CROSSPOINT_VERSION to
"{version}-mod+{git_hash}" via a pre-build script. Usage: `pio run -e mod -t upload`

Co-authored-by: Cursor <cursoragent@cursor.com>

scripts/inject_mod_version.py

@@ -0,0 +1,15 @@
+Import("env")
+import subprocess
+
+config = env.GetProjectConfig()
+version = config.get("crosspoint", "version")
+
+result = subprocess.run(
+    ["git", "rev-parse", "--short", "HEAD"],
+    capture_output=True, text=True
+)
+git_hash = result.stdout.strip()
+
+env.Append(
+    BUILD_FLAGS=[f'-DCROSSPOINT_VERSION=\\"{version}-mod+{git_hash}\\"']
+)