chore(config): enforce zdiff3 conflict style via postinstall

Add a new setup script to configure `merge.conflictStyle=zdiff3` locally
within the repository. This ensures that the common ancestor is visible
during merge conflicts, which is required for structural resolution by
`mergiraf` and simplifies manual conflict resolution.

The configuration is applied during `bun install` via the `postinstall`
hook and remains as a fallback for the upstream merge utilities.

script/setup-git.ts

@@ -0,0 +1,21 @@
+#!/usr/bin/env bun
+
+/**
+ * Configures repo-local git settings for all contributors.
+ *
+ * `merge.conflictStyle=zdiff3` makes conflict markers include the common
+ * ancestor (|||||||) alongside ours/theirs. That base section is what
+ * mergiraf's syntax-aware resolution feeds on during upstream opencode
+ * merges (see script/upstream/merge.ts) and it makes manual resolution
+ * dramatically easier than the default 2-way `merge` markers.
+ *
+ * Runs from `postinstall`. Safe to re-run — `git config` is idempotent.
+ * Guarded so tarball / docker installs without a `.git` don't fail.
+ */
+
+import { $ } from "bun"
+
+const inside = await $`git rev-parse --is-inside-work-tree`.nothrow().quiet()
+if (inside.exitCode !== 0) process.exit(0)
+
+await $`git config --local merge.conflictStyle zdiff3`.quiet()