Log without colour codes when the output is not a terminal

Under systemd the log goes to the journal, where ANSI escape codes are
noise. Colours are kept for a person at a terminal.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

bin/bgp/src/logging.rs

@@ -1,5 +1,7 @@
 //! What the daemon logs, and how much.
 
+use std::io::IsTerminal;
+
 use clap::Args;
 use tracing::Level;
 use tracing_subscriber::EnvFilter;
@@ -23,7 +25,12 @@ impl LoggingArgs {
         let filter = EnvFilter::builder()
             .from_env_lossy()
             .add_directive(format!("{OURS}={}", self.level).parse()?);
-        tracing_subscriber::fmt().with_env_filter(filter).init();
+        // Colours for a person at a terminal; plain text for the journal
+        // or a file, where escape codes are noise.
+        tracing_subscriber::fmt()
+            .with_env_filter(filter)
+            .with_ansi(std::io::stdout().is_terminal())
+            .init();
         Ok(())
     }
 }