fix(rules): YAML plain-scalar secret rule is too broad/narrow in edge cases #625

From the #621 review. (1) secret: Secret1Type (an identifier containing a digit) in .ts/.py is flagged by the unquoted-YAML rule: scope it to YAML-like files (extension or document shape) instead of every language. (2) All-alphabetic unquoted secrets (password: correcthorsebattery) and flow-style { password: hunter2hunter2 } are not flagged. (3) Unspaced password:=os.Getenv("X") can match the new Go := branch via the existing [=:] alternative. (4) SQL PASSWORD "..." with double quotes is not detected.