fix(docs): guard the checksum verification example against a grep miss

A grep miss piping empty input into the checker isn't safe to skip -
sha256sum -c - (the Linux equivalent) exits 0 on empty input, so a
typo'd filename would silently "pass" without checking anything at
all. Guard with grep -q ... && first. Also trims the accompanying
footnote back down to the platform-command difference alone - the
rationale belongs here in the commit message, not as an implementation
essay in the Readme itself.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: bougyman

Readme.adoc

@@ -46,11 +46,11 @@ standalone executables with no Erlang/Elixir install required.
 ----
 $ curl -sLO https://github.com/rubyists/linear-cli-ex/releases/latest/download/lc_macos_aarch64
 $ curl -sLo SHA256SUMS https://github.com/rubyists/linear-cli-ex/releases/latest/download/SHA256SUMS
-$ grep lc_macos_aarch64 SHA256SUMS | shasum -a 256 -c - <1>
+$ grep -q lc_macos_aarch64 SHA256SUMS && grep lc_macos_aarch64 SHA256SUMS | shasum -a 256 -c - <1>
 $ chmod +x lc_macos_aarch64
 $ sudo mv lc_macos_aarch64 /usr/local/bin/lc
 ----
-<1> Every release also publishes a `SHA256SUMS` file alongside the binaries - verify your download matches (by its original filename, before renaming/moving it) before running it. macOS ships `shasum -a 256`, not `sha256sum` - on Linux, use `sha256sum -c -` instead.
+<1> Linux: `sha256sum -c -` instead of `shasum -a 256 -c -`.
 
 NOTE: A Homebrew tap is planned but not yet available.