Reject scan --workers below 1 as a usage error (closes #10)

A --workers value of 0 or less used to be quietly raised to 1, so a
typo ran the whole scan on one worker with nothing on stderr to say
why. scan now refuses it before anything is scanned: one line on
stderr and exit 2, like the other usage errors. The clamp in runScan
is gone, and README states the rule and the default.

Model: opus-5-5

README.md

@@ -566,7 +566,9 @@ Rules for the walk:
 Concurrency: the walk phase (which also stats files), the hash phase,
 and the content phase each use a worker pool of `--workers` workers
 (default `runtime.NumCPU()`); the walk parallelizes across
-directories, hashing across files. All three phases are seek-bound on
+directories, hashing across files. `--workers` must be at least 1: a
+smaller value is a usage error, reported in one line on stderr with
+exit 2 before anything is scanned. All three phases are seek-bound on
 spinning disks, so raising `--workers` well past the core count can
 help on pools with many spindles. The main goroutine owns
 partitioning, database writes, and progress rendering; progress
@@ -761,8 +763,9 @@ Additional requirements:
     cannot be created/opened/read/written, a missing database for
     `report`/`trees`, stdout write failure), or a `scan` stopped by
     `SIGINT` or `SIGTERM` (see below).
--   `2`: usage error (including `scan` with no `PATH` operand and
-    `report`/`trees` with any positional argument).
+-   `2`: usage error (including `scan` with no `PATH` operand, `scan`
+    with `--workers` below 1, and `report`/`trees` with any positional
+    argument).
 
 A stdout write failure, such as a full disk, is reported in one line on
 stderr and exits 1. Two cases never reach sfdupes as a failed write: