Report the running build's version to the master

The version is the checkout's HEAD commit (short hash + commit time),
read once at startup — updates arrive as `git pull`, so there is no
release number to bump. It rides on the socket hello, socket reports,
and the HTTP report fallback; the field is optional in the protocol so
older masters ignore it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/version.ts

@@ -0,0 +1,43 @@
+/**
+ * The version of the running build.
+ *
+ * Updates arrive as `git pull` (scripts/update-check.mjs), so the checkout's
+ * HEAD commit *is* the version — there is no release number to bump and
+ * nothing for anyone to forget. The commit time rides along because hashes
+ * don't order: it is what lets the master say which of two different
+ * versions is the stale one.
+ *
+ * Read once and cached: the update task restarts the process after every
+ * pull, so HEAD cannot move under a running build — and when it briefly does
+ * (pull landed, restart pending), the cached value still describes the code
+ * that is actually executing.
+ */
+
+import { execFileSync } from 'node:child_process';
+
+export interface BuildVersion {
+    /** Short commit hash of HEAD, e.g. "c493796". */
+    commit: string;
+    /** ISO commit time of HEAD — orders versions, which hashes cannot. */
+    time: string;
+}
+
+let cached: BuildVersion | null | undefined;
+
+/** null when this isn't a git checkout or git isn't installed — both mean
+ *  "no version to report", and the master shows the machine as an old build. */
+export function getBuildVersion(): BuildVersion | null {
+    if (cached !== undefined) return cached;
+    try {
+        const out = execFileSync('git', ['log', '-1', '--format=%h %cI'], {
+            cwd: process.cwd(),
+            encoding: 'utf8',
+            stdio: ['ignore', 'pipe', 'ignore'],
+        }).trim();
+        const [commit, time] = out.split(' ');
+        cached = commit && time ? { commit, time } : null;
+    } catch {
+        cached = null;
+    }
+    return cached;
+}