Move the command to cmd/sfdupes and the code to internal/sfdupes (closes #157)

`go install sneak.berlin/go/sfdupes/cmd/sfdupes@...` found no package: the command was built from `package main` in the repository root. It now lives in `cmd/sfdupes/main.go`, whose one call is `sfdupes.Main()`, and every other Go file moved with `git mv` to `internal/sfdupes`, as `REPO_POLICIES.md` and the Go style guide require. The `Makefile` builds `./cmd/sfdupes` and sets `Version` by its new import path. README "Install" and `TODO.md` give the new paths; the fleet-written "flat layout" divergence is gone.

Rule suppressed: each test file carries `//nolint:testpackage`, since the tests call unexported functions throughout.

Model: opus-5-5

cmd/sfdupes/main.go

@@ -0,0 +1,29 @@
+// Command sfdupes quickly identifies candidate duplicate files across
+// very large filesystems without reading every byte of every file.
+// Files are considered duplicates when their sizes are equal and they
+// agree on three SHA-256 hashes: of the first 64 KiB, of the last 64
+// KiB, and of the content. The content hash covers the whole file under
+// 100 MiB, and 1 MiB at each gigabyte at 100 MiB or more, so a large
+// file is never read in full. scan maintains a persistent SQLite
+// database of file signatures (SFDUPES_DATABASE, default
+// /var/lib/sfdupes/db.sqlite) that the other subcommands read.
+//
+// Usage:
+//
+//	sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
+//	sfdupes report > dupes.tsv
+//	sfdupes trees > dupetrees.tsv
+//	sfdupes info
+//	sfdupes dwim
+//	sfdupes --version
+//
+// See README.md for the complete specification.
+package main
+
+import (
+	"os"
+
+	"sneak.berlin/go/sfdupes/internal/sfdupes"
+)
+
+func main() { os.Exit(sfdupes.Main()) }