feat(sync): attach local folders to existing My files folders

docs/CHANGELOG.md

@@ -11,6 +11,11 @@ scratch (user data in `staging/` and `recovery/` is never touched by this).
 
 ## [Unreleased]
 
+### Added
+
+- `pdfs sync attach PATH --to REMOTE` connects an existing local directory to an existing
+  My files folder. The first sync preserves one-sided files and differing versions (#67).
+
 ## [3.7.0] — 2026-11-02
 
 Schema: **44**, unchanged.

docs/CLI.md

@@ -79,6 +79,7 @@ commands are for scripts and for cases where the mount is not available.
 
 | Command | Description |
 |---|---|
+| `pdfs sync attach PATH --to REMOTE` | Merge a real local folder with an existing My files folder, keeping both versions on initial conflicts. |
 | `pdfs sync add PATH` | Back up a local folder under this computer and keep it in sync. |
 | `pdfs sync list` | List synced folders with their IDs and state. |
 | `pdfs sync rm ID [--delete-remote]` | Stop syncing. Local files stay; `--delete-remote` also removes the copy on Proton Drive. |

docs/SYNCED_FOLDERS.md

@@ -169,3 +169,19 @@ The version you do not keep goes to the Trash, so a wrong choice can be undone.
   `pdfs sync resume` continues at once.
 - **Limit bandwidth** under **Preferences → General → Network**, or with
   `pdfs sync limit --up 2M --down 10M`. `0` removes a limit.
+
+## Attach an existing My files folder
+
+Run `pdfs sync attach /path/to/local --to Work` to merge a real local directory
+with the existing `Work` folder in My files. `--to` also accepts an absolute path
+inside the My files mount. Both folders must already exist.
+
+The local directory stays a normal mirrored directory. The first pass copies files
+found on only one side, adopts identical files, and keeps a conflict copy when
+versions differ; it does not propagate deletions. Later passes use ordinary
+bidirectional sync, including deletions and the existing wipe guards.
+
+The My files root, shared folders, overlapping sync roots, and local paths inside
+or containing the cloud mount are refused. Use `pdfs sync list` to find the new
+folder's ID and monitor its state. `pdfs sync rm ID` stops syncing; adding
+`--delete-remote` also trashes the linked cloud folder.