[FEATURE]: let opencode run ignore an unused inherited stdin pipe

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

opencode run reads non-TTY stdin to EOF before starting a prompt. That is the right default for intentional input such as:

cat context.txt | opencode run "review this"

The problem is that some supervisors give every child a pipe and keep its writer open, even when the command has no stdin payload. The prompt then never starts. This minimal Python control retains the pipe deliberately; changing PIPE to DEVNULL makes the same command progress to the expected invalid-model error:

import subprocess

args = ["opencode", "run", "--model", "invalid/provider-model", "reply with OK"]

def run(stdin):
    child = subprocess.Popen(args, stdin=stdin)
    try:
        return f"exit {child.wait(timeout=5)}"
    except subprocess.TimeoutExpired:
        child.terminate()
        child.wait()
        return "still waiting after 5s"

print("open pipe:", run(subprocess.PIPE))
print("closed input:", run(subprocess.DEVNULL))

Could run expose an explicit opt-out, for example --no-stdin? It would skip only the stdin read for that invocation; the existing default and pipe + positional prompt behavior would not change. I have a small implementation and subprocess tests ready if this interface is acceptable.

The symptom also appeared in #11891, which documented DEVNULL as a caller-side workaround. Related #25508 and merged #16300 show why silently ignoring piped input is not safe. When the caller controls descriptors, DEVNULL, stdin: "ignore", or closing the writer remains the workaround.