fix(read): make head/tail rewrites faithful to the native commands

The hook rewrites `head -N FILE` to `rtk read FILE --max-lines N`, but
`--max-lines` routes through smart_truncate, which keeps a non-important
line only while kept_lines < max_lines / 2. Every rewritten `head -N`
therefore showed about half the requested lines, and `head -1` showed
none at all. Bare `head FILE` rewrote to an unbounded read of the whole
file, which a test asserted as intended.

Add `--head-lines N` for an exact first-N-lines window and point the
head rewrites at it, leaving `--max-lines` preview semantics unchanged
for existing callers. Both windows now slice on byte offsets at the Nth
newline instead of round-tripping through lines(), so CRLF endings and
an unterminated final line survive. The shipped `--tail-lines` path had
the same normalization loss. `--max-lines 0` returned a marker with no
content and underflowed max_lines - 1 on usize; it now returns empty,
matching `--tail-lines 0`.

Gate the rewrites on is_single_file_operand, an allowlist of characters
that cannot change an operand's word count or turn it into an option. A
whitespace-delimited token is not a shell operand: `head -n 1 *.rs`
expanded to sixteen files and lost the `==> name <==` banners,
`head -n 1 --help` printed rtk's own help, and a leading `#` opened a
comment that swallowed the appended flags. Uncertain operands stay with
the native binary, which costs coverage on quoted literals such as
'literal*.txt' and keeps behavior correct.

Separate a stripped trailing redirect from the appended flag value.
`head -n 1 a>b` produced `--head-lines 1>b`, which the shell reads as an
fd-1 redirect, leaving the flag without a value and the redirect target
empty.