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/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)