Add scan --exclude PATH (closes #163)

The walk neither enters nor records an --exclude path or anything
below it, and prints "scan: skipped PATH (--exclude)" for each one it
reaches. The flag repeats. Each value is resolved to its real path, as
an operand is. The records below an excluded path are kept, as for a
default exclude. An --exclude path that does not exist, or an operand
at or below one, is a fatal error before the scan starts.

Model: opus-5-5

TODO.md

@@ -29,6 +29,11 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- 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 (2026-11-10,
+  https://git.eeqj.de/sneak/sfdupes/issues/163)
+
 - The Go code moved out of the repository root. `cmd/sfdupes/main.go` is the
   command, and it calls `Main` in the package `internal/sfdupes`. `main.go`
   became `internal/sfdupes/cli.go`. Every other file kept its name. The install