Move the command line from cmd/bsdaily into internal/cli (closes #8)

cmd/bsdaily/main.go now only passes Version to cli.Main and exits with
the status it returns, as REPO_POLICIES.md requires of cmd/. The cobra
command, the flag rules and the date parsing moved unchanged into
internal/cli, so flags, help text, error messages, exit status and the
first log line stay as they were. The fallback to dev for an empty
version moved with them into cli.Main.

parseTargetDates is exported as ParseTargetDates so that a table test
outside the package can reach it; the test never touches the filesystem
or runs the extraction. README describes the new layout and TODO.md
records the step.

Model: opus-5-5

TODO.md

@@ -23,7 +23,6 @@ tests for the extraction, verification, and atomic-publish paths.
   parsing) from `cmd/bsdaily` into `internal/cli`, with unit tests for the flag
   rules and the dates; `cmd/bsdaily/main.go` is now a single call into it
   (https://git.eeqj.de/sneak/bsdaily/issues/8).
-
 - 2026-10-06: A plain `docker build .` and a host `make` build stamp the git tag
   or short commit into the binary, which `bsdaily` logs on the first line of
   every run and prints with `--version`

internal/cli/dates_test.go

@@ -54,14 +54,18 @@ func TestParseTargetDates(t *testing.T) {
 func TestParseTargetDatesErrors(t *testing.T) {
 	t.Parallel()
 
-	// wantErr is how the error message starts: for a malformed date, the
-	// date parser's own explanation follows it.
+	const dateExclusive = "--date and --from/--to are mutually exclusive"
+
+	// wantErr is the whole error message. With startsWith set it is only how
+	// the message starts: for a malformed date, the date parser's own
+	// explanation follows it.
 	tests := []struct {
-		name    string
-		date    string
-		from    string
-		to      string
-		wantErr string
+		name       string
+		date       string
+		from       string
+		to         string
+		wantErr    string
+		startsWith bool
 	}{
 		{
 			name: "from after to", from: "2026-03-02", to: "2026-03-01",
@@ -71,28 +75,36 @@ func TestParseTargetDatesErrors(t *testing.T) {
 		{name: "to without from", to: "2026-04-02", wantErr: "--to requires --from"},
 		{
 			name: "date with from", date: "2026-01-05", from: "2026-01-06",
-			wantErr: "--date and --from/--to are mutually exclusive",
+			wantErr: dateExclusive,
+		},
+		{
+			name: "date with to", date: "2026-01-13", to: "2026-01-14",
+			wantErr: dateExclusive,
 		},
 		{
 			name: "date with from and to", date: "2026-01-07",
 			from: "2026-01-08", to: "2026-01-09",
-			wantErr: "--date and --from/--to are mutually exclusive",
+			wantErr: dateExclusive,
 		},
 		{
 			name: "malformed date", date: "27.06.2026",
-			wantErr: `invalid --date "27.06.2026" (expected YYYY-MM-DD): `,
+			wantErr:    `invalid --date "27.06.2026" (expected YYYY-MM-DD): `,
+			startsWith: true,
 		},
 		{
 			name: "date that does not exist", date: "2026-02-30",
-			wantErr: `invalid --date "2026-02-30" (expected YYYY-MM-DD): `,
+			wantErr:    `invalid --date "2026-02-30" (expected YYYY-MM-DD): `,
+			startsWith: true,
 		},
 		{
 			name: "malformed from", from: "2026-1-10", to: "2026-01-11",
-			wantErr: `invalid --from "2026-1-10" (expected YYYY-MM-DD): `,
+			wantErr:    `invalid --from "2026-1-10" (expected YYYY-MM-DD): `,
+			startsWith: true,
 		},
 		{
 			name: "malformed to", from: "2026-01-12", to: "tomorrow",
-			wantErr: `invalid --to "tomorrow" (expected YYYY-MM-DD): `,
+			wantErr:    `invalid --to "tomorrow" (expected YYYY-MM-DD): `,
+			startsWith: true,
 		},
 	}
 
@@ -101,8 +113,14 @@ func TestParseTargetDatesErrors(t *testing.T) {
 			t.Parallel()
 
 			_, err := cli.ParseTargetDates(tt.date, tt.from, tt.to)
-			if err == nil || !strings.HasPrefix(err.Error(), tt.wantErr) {
-				t.Errorf("error = %v, want one starting %q", err, tt.wantErr)
+
+			switch {
+			case err == nil:
+				t.Errorf("no error, want %q", tt.wantErr)
+			case tt.startsWith && !strings.HasPrefix(err.Error(), tt.wantErr):
+				t.Errorf("error = %q, want one starting %q", err, tt.wantErr)
+			case !tt.startsWith && err.Error() != tt.wantErr:
+				t.Errorf("error = %q, want %q", err, tt.wantErr)
 			}
 		})
 	}