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)
 

internal/web/router.go

@@ -29,6 +29,7 @@ func NewRouter(s *Server) chi.Router {
 			protected.Post("/apply", s.handleApply)
 			protected.Get("/apply/log", s.handleApplyLog)
 			protected.Get("/system/status", s.handleSystemStatus)
+			protected.Get("/version", s.handleVersion)
 			protected.Put("/auth/password", s.handleChangePassword)
 
 			protected.Get("/import/status", s.handleImportStatus)

internal/web/version.go

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