Leave caches out of the walk by default (closes #181)

The default excludes now cover caches that programs and package
managers fill again on their own. On Linux: /var/cache and, in /root
and in each home in /home, .cache, .npm, .cargo/registry and
go/pkg/mod. On macOS: /private/var/folders, /Library/Caches and, in
/private/var/root and in each home in /Users, Library/Caches plus the
same four. README and scan --help list them; --no-default-excludes
still turns them all off.

Model: opus-5-5

TODO.md

@@ -29,6 +29,13 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The default excludes cover caches as well. On Linux they add `/var/cache`. In
+  `/root` and in each home in `/home` they add `.cache`, `.npm`,
+  `.cargo/registry` and `go/pkg/mod`. On macOS they add `/private/var/folders`
+  and `/Library/Caches`. In `/private/var/root` and in each home in `/Users`
+  they add `Library/Caches` and the same four. `--no-default-excludes` still
+  turns them all off (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/181)
+
 - `dwim` takes the `scan` flags `--workers`, `-x`, `--no-default-excludes` and
   `--exclude`, with the same meaning. Both commands register and check them with
   the same code (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/162)