[TE-5578] Use Fprintln for unformatted base-branch warning

The base-branch resolution warning in `autoCollectGitMetadata` calls
`fmt.Fprintf` but contains no formatting verbs, and embeds a trailing
"\n" in the format string. That is exactly the shape `Fprintln` exists
for: no verbs, implicit newline. Switch to `Fprintln` and drop the
explicit "\n".

Pure style change; no behaviour difference on stderr.

Addresses review feedback on PR #480.

Author: gchan

internal/command/plan.go

@@ -168,8 +168,8 @@ func autoCollectGitMetadata(ctx context.Context, cfg *config.Config, runner git.
 	remote := cfg.Remote
 	baseBranch, err := git.ResolveBaseBranch(ctx, runner, explicit, remote)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "Warning: could not resolve base branch for diff metadata. "+
-			"Set --metadata base_branch=<branch> if your repo uses a non-standard default branch.\n")
+		fmt.Fprintln(os.Stderr, "Warning: could not resolve base branch for diff metadata. "+
+			"Set --metadata base_branch=<branch> if your repo uses a non-standard default branch.")
 	} else {
 		debug.Printf("auto-detected base branch: %s", baseBranch)
 	}