fix: configure ours merge driver for pixi.lock auto-resolve

merge=ours in .gitattributes is not a built-in git merge driver—it
needs an explicit driver definition. Without it, git falls back to the
text driver and reports conflicts on pixi.lock.

Add `git config merge.ours.driver true` to devcontainer postCreateCommand
so the driver is configured on container creation.

Author: blooop

.devcontainer/devcontainer.json

@@ -47,5 +47,5 @@
         "source=${localEnv:HOME}/.config/gh,target=/home/vscode/.config/gh,type=bind",
         "source=${localEnv:HOME}/.claude,target=/home/vscode/.claude,type=bind"
     ],
-    "postCreateCommand": "sudo chown vscode .pixi && pixi install"
+    "postCreateCommand": "sudo chown vscode .pixi && git config merge.ours.driver true && pixi install"
 }