[feat] CLI-02/C-3: stdin/pipe reading on the sys module

[vt128]

CLI-02 / C-3 — read piped data from stdin

Per the maintainer's call (data comes from stdin; the script comes from a file
or -c), let scripts consume piped data from standard input. Three
additions to the sys module, mirroring Python's sys.stdin:

function	behaviour
sys.read()	all of stdin as a string
sys.lines()	a lazy iterator over stdin lines (trailing CR/LF trimmed) — a large stream is consumed line by line, not buffered whole; one-shot, like for line in sys.stdin
sys.isatty()	whether stdin is a terminal vs a pipe/file (branch interactive vs piped)

load("sys", "lines")
for line in lines():
    print(line.upper())

$ printf 'foo\nbar\n' | ./starcli upper.star
FOO
BAR

No new module or capability — sys is already wired and classified
CapProcess. Tests redirect os.Stdin through a pipe to cover
read/lines (including a final line with no newline)/isatty. README
documents it. Coverage 75.8% (≥ 65); Docker golang:1.22 green.

This rounds out CLI-02 (STAR-19): check (#9) + args (#11) + stdin now all landed.