Document Milestone 3 deliverables and SSH prerequisites

docs/PROGRESS.md

@@ -43,3 +43,20 @@ Delivered:
 
 Git:
 - Tag: `v0-m2-done`
+
+## Milestone 3 - Real SSH Backend and Session Controls
+
+Status: Completed
+
+Delivered:
+- Backend architecture introduced (`SessionBackend` + protocol-specific implementations)
+- Worker-thread backend execution for connection lifecycle operations
+- Real SSH process backend (`ssh`) with connect/disconnect/reconnect
+- Unsupported protocol backend with explicit not-implemented messaging (RDP/VNC)
+- Session tab controls: `Connect`, `Disconnect`, `Reconnect`, `Copy Error`
+- Connect-time credential flow (password prompt / private-key path selection)
+- Session event log pane with timestamps and user-friendly error mapping
+- SQLite profile schema migration for `private_key_path` and `known_hosts_policy`
+
+Git:
+- Tag: `v0-m3-done`