parseIssueArg multiline IDs

Trap: passing all multiline issue input to validateResourceId() looks strict, but command substitution, appended notes, or multiple IDs then produce a cryptic newline error. Fix: parseIssueArg() selects the first nonblank trimmed line because an issue identifier is one atomic line; never join lines as wrapped URLs do. Spaces in project display names are unaffected because line splitting targets control characters, not spaces.