* refactor: remove managed/standalone mode distinction from codebase
Standalone mode is deprecated; managed mode is now the only mode.
Remove redundant "managed mode" qualifiers from comments, docs,
and error messages. Error strings now reference "database stores"
instead of "managed mode" for clarity.
* improve(onboard): streamline onboard process and env setup
Simplify onboard wizard, extract helpers to dedicated file,
update env example and entrypoint for default managed mode,
clean up prepare-env script, update i18n catalogs.
- Update go.mod and Dockerfile to Go 1.26
- Apply `go fix ./...` stdlib modernizations across 170+ files
- Add `go fix` to post-implementation checklist in CLAUDE.md
- Fix go fix misapplied rewrite in loop_history.go
