Add git fetch prune to improve Docker container initialization

This change modifies the agent initialization to run 'git fetch --prune sketch-host'
instead of 'git fetch sketch-host' when running in Docker. This ensures that
leftover remote branches from the sketch host are properly cleaned up.

Co-Authored-By: sketch <hello@sketch.dev>
diff --git a/loop/agent.go b/loop/agent.go
index 9858715..910b4cc 100644
--- a/loop/agent.go
+++ b/loop/agent.go
@@ -655,7 +655,7 @@
 		if out, err := cmd.CombinedOutput(); err != nil {
 			return fmt.Errorf("git remote add: %s: %v", out, err)
 		}
-		cmd = exec.CommandContext(ctx, "git", "fetch", "sketch-host")
+		cmd = exec.CommandContext(ctx, "git", "fetch", "--prune", "sketch-host")
 		cmd.Dir = ini.WorkingDir
 		if out, err := cmd.CombinedOutput(); err != nil {
 			return fmt.Errorf("git fetch: %s: %w", out, err)
@@ -1505,7 +1505,7 @@
 	if out, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("git stash: %s: %v", out, err)
 	}
-	cmd = exec.CommandContext(ctx, "git", "fetch", "sketch-host")
+	cmd = exec.CommandContext(ctx, "git", "fetch", "--prune", "sketch-host")
 	cmd.Dir = workingDir
 	if out, err := cmd.CombinedOutput(); err != nil {
 		return fmt.Errorf("git fetch: %s: %w", out, err)