fix(newos4): coreutils flags + loop-guard arg key, both found by the agent's live run (v4.6.49)

Agent session surfaced two real defects:

1. The A4 prompt told it to use grep -n / sed -n 'N,Mp' / wc -l / head -N — none of those flags
   parsed: grep treated '-n' as the pattern (6/6 variants failed), wc read '-l' as a filename,
   head looked for a file named '-2', sed had no range parser. Fixed the tools, not just the text:
   grep now accepts -n/-i/-c/-v/-l (line numbers were already printed), wc accepts -l/-w/-c,
   head accepts -N and -n N, sed accepts Np and N,Mp, tail no longer prints a blank line for a
   trailing newline. Prompt re-ranked to the working forms.

2. Loop guard keyed calls by tool name plus the raw arguments field. When arguments was already
   an object (parsed upstream), the key became 'run_command:[object Object]', collapsing distinct
   batched commands into one signature and producing false "called 3x" warnings. Now the key is
   JSON.stringify(arguments) whenever it is not a string (3 sites: sig, _sigR, _sigD).

Verified live v4.6.49: grep -n/-c, wc -l, head -2, sed -n '2,3p', tail -n 1 all correct.

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