Two-part fix for the dashboard fd exhaustion reported in #81547:
1. Raise RLIMIT_NOFILE soft limit on startup (before uvicorn binds).
macOS defaults to 256 for LaunchAgent processes — too tight for the
dashboard which opens 3 fds (db+wal+shm) per SessionDB per request
across all profiles. After days of polling the soft limit exhausts
and every os.listdir/open raises OSError [Errno 24]. The helper raises
to the hard limit (or minimum 4096), matching the reporter's ulimit
workaround. No-op on Windows (no resource module).
2. Replace bare Path.iterdir() with context-managed os.scandir() in four
dashboard hot paths: _fallback_profile_dicts, file manager list,
checkpoint listing, and plugin discovery. iterdir() returns a
generator that holds an open directory fd until fully consumed; if
an exception interrupts iteration the fd leaks. os.scandir() is an
explicit context manager that guarantees close on exit, following
the same idiom already used in /api/fs/list.
Tests: 6 passed, 3 skipped (resource-module tests skip on Windows).
Michael Gannotti