fix: exit 0 when a downstream reader closes stdout early

[varuntej07]

Problem

Piping graphify into a reader that stops early (head, PowerShell
Select-Object -First N, a pager that quits) closes stdout before we finish
writing. The interpreter's shutdown flush then hits the closed pipe and crashes:

Exception ignored on flushing sys.stdout:
OSError: [Errno 22] Invalid argument

The process exits nonzero (120 on POSIX). A downstream head closing the pipe
is normal, so graphify should exit 0 the way foo | head does.

Fix

Wrap the CLI entry point: catch BrokenPipeError and OSError with EINVAL or
EPIPE (Windows reports a closed pipe as EINVAL), redirect stdout to devnull
so the shutdown flush stays quiet, and exit 0. Unrelated OSErrors still
propagate, so real failures such as a missing graph.json are not masked.

Verification

A reader that reads one line then closes the pipe:

before: returncode=120, "Exception ignored on flushing sys.stdout"
after:  returncode=0,   clean

Adds tests/test_broken_pipe_exit.py (5 tests covering the exit-code contract,
including that non-pipe errors still propagate). Existing CLI suites pass.