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/handlers_system.go

@@ -37,6 +37,13 @@ func (s *Server) handleSystemStatus(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, status)
 }
 
+func (s *Server) handleVersion(w http.ResponseWriter, r *http.Request) {
+	writeJSON(w, http.StatusOK, map[string]string{
+		"version": Version,
+		"commit":  Commit,
+	})
+}
+
 func (s *Server) collectDiskUsage() []diskUsage {
 	entries := make(map[string]*diskUsage)