fix(git): window blob-show in pipes too, matching diff/log filters

The is_terminal gate disabled blob windowing whenever stdout was not a TTY.
Since rtk is invoked by the agent hook with stdout piped, that left the
feature inert in its primary scenario (0 token savings on a piped
git show <rev>:<path>), while diff/log/status all compact in a pipe. It was
also unsound as a safety guard: CI agents hand rtk a pseudo-TTY (see
hooks/init.rs), so is_terminal() does not reliably tell a human from a
downstream consumer.

Window large text blobs regardless of TTY and rely on the tee-independent
'git show <rev>:<path> | tail -n +N' recovery hint for exact reconstruction --
the same tradeoff 'git log | grep' already accepts. Small (<=budget) and
binary blobs still pass through byte-identically. Also skip colons-only
tokens (':' , '::') in is_blob_show_arg so they route to git rather than the
blob window.