fix: discover_local_files infinite loop on symlink cycles

[noahgift]

Bug

discover_local_files() in src/bin/copia.rs:227 uses path.is_dir() which follows symlinks. A symlink pointing to an ancestor directory creates an infinite traversal loop, consuming unbounded memory.

Reproduction

mkdir -p /tmp/copia-test/a/b
ln -s /tmp/copia-test /tmp/copia-test/a/b/loop
copia sync /tmp/copia-test user@host:/dest
# Hangs forever, OOM

Fix

Skip symlinked directories:

if path.is_dir() && !path.is_symlink() {
    dirs.push(path);
}

Or track visited inodes with a HashSet<(u64, u64)> for (dev, ino) pairs.