If you have an iPhone (or iPad) and a Mac signed into the same Apple ID, you already have everything you need. iCloud Drive is built into both. You just need to point Keepance at the right folder.
Step 1: Make sure iCloud Drive is on
On your Mac
Open System Settings, click your Apple ID at the top, then click iCloud.
Make sure iCloud Drive is turned on. If it isn't, flip the toggle and wait a moment for it to sync.
On your iPhone
Open the Files app. If you've never used it, it's the blue folder icon that ships with iOS.
Tap Browse at the bottom. You should see iCloud Drive in the list of locations. If it's missing, go to Settings → [your name] → iCloud → iCloud Drive and turn it on.
Step 2: Point Keepance at iCloud Drive
In Keepance on your Mac
Open Keepance. From the menu, choose File → Open Workspace (or click the workspace name in the sidebar to switch).
In the folder picker, navigate to your iCloud Drive folder. The full path is: ~/Library/Mobile Documents/com~apple~CloudDocs/
Create a new folder called Keepance inside iCloud Drive (or pick an existing folder if you already have one).
Select that folder as your workspace and click Open.
From this moment on, every file Keepance writes lives inside iCloud Drive and syncs automatically.
[Screenshot: Mac Keepance with workspace pointing at iCloud Drive folder]
TODO: real screenshot of Keepance's workspace selector pointing at iCloud Drive.
Step 3: Wait for sync
The first sync can take anywhere from a few seconds to a few minutes depending on how much is in your workspace. You can watch progress in the Finder: open your iCloud Drive folder and look for the cloud icon next to each file. A solid icon means it's uploading; no icon means it's done.
If your workspace is brand new and only has a handful of Markdown files, sync should finish in well under a minute.
Step 4: Open your workspace on iPhone
On your iPhone, open the Files app.
Tap Browse, then iCloud Drive, then your Keepance folder.
You'll see your workspace exactly as it is on your Mac. Tap any .md file to read it.
[Screenshot: iPhone Files app showing the synced Keepance workspace]
TODO: real screenshot of iPhone Files browsing the Keepance folder in iCloud Drive.
iOS Files renders Markdown as plain text. If you want pretty rendering on the phone, install a free Markdown reader from the App Store (Taio, 1Writer, and iA Writer all work well) and point it at the same iCloud Drive folder.
Limitations to know
Treat your phone as read-only for now. If you edit a file on both your Mac and your phone within a few seconds of each other, iCloud Drive can produce a conflict copy named something like VISION 2.md. To avoid this, pick one device at a time as the editor.
iCloud free tier is 5 GB. A typical Keepance workspace is far under that. If you hit the limit, you can either delete other iCloud content or pay for iCloud+ (starts at 99 cents per month for 50 GB).
Sync delay is normally a few seconds, sometimes longer. If you edit on the desktop and immediately open the phone, you might see the previous version for a moment. Pull down on the Files screen to force-refresh.
Wiki-link navigation doesn't work in Files. The phone shows file contents as plain text. Wiki-links and backlinks are a Keepance feature; they round-trip to the Mac when you tap a file there.
What's next
The iCloud Drive workaround is the simplest mobile path on Apple devices. If you also want to sync to non-Apple devices, see the Dropbox guide or the Syncthing guide.