Keep the database schema at version 1 (closes #61)

sfdupes is pre-1.0, with no installed base and no databases anywhere,
so the schema is changed in place and its version stays 1.
schemaVersion goes back to 1; the six-column files table, content
included, is the version 1 schema. The check that stops on a database
with any other version stays. README.md and TODO.md no longer describe
a version 2 or rejecting and rescanning version 1 databases. The
main.go package comment still described 1024-byte end windows and said
full file contents are never read; it now describes the hashes the
code computes.

Model: opus-5-5

main.go

@@ -1,10 +1,13 @@
 // Command sfdupes quickly identifies candidate duplicate files across
-// very large filesystems without reading full file contents. Files are
-// considered duplicates when they have identical size, identical SHA-256
-// of their first 1024 bytes, and identical SHA-256 of their last 1024
-// bytes. scan maintains a persistent SQLite database of file signatures
-// (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite) that the
-// reporting subcommands read.
+// very large filesystems without reading every byte of every file.
+// Files are considered duplicates when their sizes are equal and they
+// agree on a short ladder of SHA-256 hashes. A file under 10 MiB is
+// hashed in full. A larger file is compared on the hashes of its first
+// and last 64 KiB and on a content hash: of the whole file when it is
+// under 50 MiB, or of gigabyte-spaced 1 MiB samples when it is 50 MiB
+// or larger. scan maintains a persistent SQLite database of file
+// signatures (SFDUPES_DATABASE, default /var/lib/sfdupes/db.sqlite)
+// that the reporting subcommands read.
 //
 // Usage:
 //