docs: correct Go version and remove unshipped SFTP browser claims

- README: build-from-source requires Go 1.25+ (matches go.mod), was 1.23+.
- docs/v2-report.md: the in-app SFTP file browser was never built (no `t`
  action, no xfer/sshx/browser packages, pkg/sftp not a dependency). Cut the
  false "shipped" claims and moved it into the Deferred section.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -106,7 +106,7 @@ that ships in `%WINDIR%\System32\OpenSSH`.
 
 ### Build from source instead
 
-Requires Go 1.23+.
+Requires Go 1.25+.
 
 ```sh
 make install     # builds and installs into $GOBIN (or $GOPATH/bin, ~/go/bin)