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, for root
and each user under /home, .cache, .npm, .cargo/registry and
go/pkg/mod. On macOS: /private/var/folders, /Library/Caches and, for
each user, 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 +29,12 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The default excludes cover caches as well: `/var/cache`, and each user's
+  `.cache`, `.npm`, `.cargo/registry` and `go/pkg/mod`, root's included. On
+  macOS they add `/private/var/folders`, `/Library/Caches` and each user's
+  `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)
+
 - New `scan` flag `--exclude PATH`, which may be repeated. The walk neither
   enters nor records an excluded path, and keeps the records below it. An
   operand at or below one is a fatal error (2026-11-11,