fix(launcher): strip trailing LF from frontmatter body

The body region rebuilt from line-split included a trailing empty
entry from the final LF the parser appends to drive its gmatch. Strip
it so 'body text\n' (the on-disk content) parses to 'body text' (the
user-visible body), which is what the launcher-test asserts.

frontmatter.lua

@@ -37,6 +37,11 @@ local function split_at_separator(content)
     for i = 2, sep_idx - 1 do json_lines[#json_lines + 1] = lines[i] end
     local body_lines = {}
     for i = sep_idx + 1, #lines do body_lines[#body_lines + 1] = lines[i] end
+    -- Drop the trailing empty entry produced by the final LF appended above,
+    -- so the body matches user expectation ('body text' rather than 'body text\n').
+    if #body_lines > 0 and body_lines[#body_lines] == "" then
+        body_lines[#body_lines] = nil
+    end
     return table.concat(json_lines, "\n"), table.concat(body_lines, "\n")
 end