chore: checkpoint current performance and UI changes

performance.py

@@ -0,0 +1,31 @@
+"""Tiny opt-in performance tracing helpers.
+
+Tracing is disabled by default and emits only timings/counts, never prompts,
+credentials, file contents, or provider payloads.
+"""
+from __future__ import annotations
+
+import logging
+import os
+import time
+from contextlib import contextmanager
+
+_LOG = logging.getLogger("cowork.performance")
+
+
+def enabled() -> bool:
+    return os.environ.get("COWORK_PERF_TRACE", "").strip().lower() in {"1", "true", "yes"}
+
+
+@contextmanager
+def span(name: str, **fields):
+    if not enabled():
+        yield
+        return
+    started = time.perf_counter()
+    try:
+        yield
+    finally:
+        elapsed = (time.perf_counter() - started) * 1000.0
+        safe = " ".join(f"{k}={v}" for k, v in fields.items())
+        _LOG.info("perf %s %.1fms%s", name, elapsed, f" {safe}" if safe else "")