feat(builder): launch options must appear in the research sources

The Valheim draft in #13 passed -maxplayers, an option Valheim doesn't have.
The server ignores unknown options, so the draft booted and passed every
boot test while its MaxPlayers setting did nothing.

build now reads the pages research based the variant on, in full, and the
drafter rejects any launch option in the startup command that none of them,
nor the image's own scripts, mention. The model gets it back to remove or
spell correctly. Options of shell tools (mkdir -p, [ -n ...]) are skipped.
The check is off when there are no sources (draft run by hand, no reader).

Against #13's 11 real sources it flags -maxplayers and nothing else.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/gbrgames/research.go

@@ -250,13 +250,18 @@ func dedupeHits(hits []searchHit) []searchHit {
 }
 
 func readPage(reader, pageURL string) (string, error) {
+	return readPageLimit(reader, pageURL, 9000)
+}
+
+// readPageLimit reads a page as text through the reader, cut to limit bytes.
+func readPageLimit(reader, pageURL string, limit int) (string, error) {
 	raw, err := httpGet(strings.TrimRight(reader, "/")+"/"+pageURL, 60*time.Second)
 	if err != nil {
 		return "", err
 	}
 	text := string(raw)
-	if len(text) > 9000 {
-		text = text[:9000] + "\n...(truncated)"
+	if len(text) > limit {
+		text = text[:limit] + "\n...(truncated)"
 	}
 	return text, nil
 }