fix the OTP activation snippet in CLAUDE.md

[next-nf]

The activation snippet in CLAUDE.md silently does nothing.

kerl list installations names the OTP 28.3 install 28.3, not otp-28.3:

$ kerl list installations
otp-29 /home/nathanf/.local/bin/otp-29
28.3 /home/nathanf/.local/bin/otp-28.3

So /^otp-28\.3 / matches no line, $(...) expands to empty, and the command
degrades to . /activate — which fails and leaves whatever OTP is on PATH
active. Builds still pass, because the default OTP happens to be usable, which
is exactly why this went unnoticed.

Changing the pattern to /^28\.3 / makes it resolve. Verified after the fix:

$ . $(kerl list installations | awk '/^28\.3 /{print $2}')/activate
$ erl -noshell -eval 'io:format("OTP ~s~n",[erlang:system_info(otp_release)]),halt().'
OTP 28

Note the install name is a local kerl choice rather than something the repo
controls, so on a machine where the 28.3 install is named otp-28.3 the old
pattern is the correct one. If that split turns out to matter across the team,
a pattern matching both (e.g. /^(otp-)?28\.3 /) would be the more portable
fix — happy to switch to that instead.

Based on main, independent of the async stack (#45 → #67). Heads-up: #45 adds
a section immediately after this code block, so rebasing that stack onto a main
containing this commit may flag a trivial overlap there; the resolution is to
keep both changes.