fix(otel): keep piped stderr diagnostics plain — fmt layer ansi follows tty

tracing_subscriber's fmt layer enables ANSI whenever the 'ansi' feature is on, regardless of destination, so redirected stderr carried SGR escapes where the pre-migration eprintln! path emitted plain bytes. Gate the layer on std::io::stderr().is_terminal().

agent-identity: unknown
agent-persona: generalist
agent-supervisor: unavailable
agent-tool: OMP
agent-tool-version: 18.0.3
agent-runtime: OMP 18.0.3
tooling-profile: dotfiles@929dc21

Author: schickling-assistant

src/telemetry.rs

@@ -12,6 +12,7 @@
 //! exports log records via an SDK logger provider. Logs emitted inside a span automatically
 //! carry its trace/span ids.
 
+use std::io::IsTerminal as _;
 use std::sync::atomic::{AtomicBool, Ordering};
 
 use opentelemetry::trace::{Span as _, Tracer as _};
@@ -291,6 +292,7 @@ fn install_subscriber<S>(
             )
             .with(
                 tracing_subscriber::fmt::layer()
+                    .with_ansi(std::io::stderr().is_terminal())
                     .with_writer(std::io::stderr)
                     .with_filter(filter),
             ),