feat: bake version into binary and display in UI sidebar

- Inject VERSION via ldflags (-X main=web.Version=) and git short SHA
- Add /api/version endpoint returning {version, commit}
- Display version in Layout sidebar footer (v0.1.6 · 8cfd405)
- Add AGENTS.md section documenting SemVer policy and bump-before-push rule
- Bump VERSION 0.1.5 -> 0.1.6 in Makefile

internal/web/version.go

@@ -0,0 +1,11 @@
+package web
+
+var (
+	Version = "dev"
+	Commit  = ""
+)
+
+func SetVersionInfo(v, c string) {
+	Version = v
+	Commit = c
+}