Fix two brew install lines that error out

`brew install gnu-sed --with-default-names` fails with "invalid option"
- Homebrew dropped per-formula option flags years ago. The flag existed
to install GNU sed as `sed` rather than `gsed`, which is now the
opposite of what we want, so drop it and note why in the comment.

`oven-sh/bun/bun` no longer resolves; bun has since moved into
homebrew-core, so install it by its plain name. This line has been
failing on every bootstrap, which is why bun was never actually
installed.

Neither aborted the script (there is no `set -e`), they just failed
their own install and carried on. Verified all 47 formulae and 6 casks
now resolve and none are deprecated or disabled.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

brew.sh

@@ -16,8 +16,8 @@ brew install coreutils
 
 # Install more system utilities
 brew install moreutils findutils
-# Install GNU `sed`
-brew install gnu-sed --with-default-names
+# Install GNU `sed` (as `gsed` - we deliberately don't shadow BSD sed, see zshrc.zsh)
+brew install gnu-sed
 
 # Install more recent versions of some macOS tools.
 brew install vim
@@ -33,7 +33,7 @@ brew install age
 brew install awscli
 brew install b3sum
 brew install bat
-brew install oven-sh/bun/bun
+brew install bun
 brew install diff-so-fancy
 brew install eza
 brew install fd