build(desktop): pin SwiftTerm to fixed revision b6ce28a

Track a specific SwiftTerm revision instead of branch main, so its
performance characteristics don't shift under us between builds.

Refs: RMI-PLEXUSONEAPP-004

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: grokify

apps/desktop/Package.swift

@@ -12,7 +12,7 @@ let package = Package(
         .executable(name: "PlexusOneDesktop", targets: ["PlexusOneDesktop"])
     ],
     dependencies: [
-        .package(url: "https://github.com/migueldeicaza/SwiftTerm.git", branch: "main"),
+        .package(url: "https://github.com/migueldeicaza/SwiftTerm.git", revision: "b6ce28a4b222b06d76a3fd44e904e00a95044d53"),
         .package(url: "https://github.com/plexusone/assistantkit-swift.git", from: "0.1.0")
     ],
     targets: [