hil, docs: reference toolchains by their official env vars, not one rig's paths

~/code/pico/pico-sdk and $HOME/code/esp-idf/export.sh are the ci rig's private layout;
written into instructions they silently stop being true on tusb, a dev PC, or any
future rig. The docs now use the variables the vendors define -- PICO_SDK_PATH for the
Pico SDK and IDF_PATH for ESP-IDF, activated explicitly as `. "$IDF_PATH/export.sh"` --
and leave where the checkouts live to each host's profile.

The variables are only useful if the shells that agents actually get can see them, and
`ssh <rig> 'cmd'` is non-interactive AND non-login: it reads no profile, and Debian's
sshd-sourced ~/.bashrc returns at the interactive guard before most of the file. The ci
rig already keeps its exports in the section ABOVE that early-return; IDF_PATH now sits
there beside PICO_SDK_PATH, and the whole chain is verified from a plain non-interactive
ssh: both variables visible, `. "$IDF_PATH/export.sh"` activates ESP-IDF v5.5.3 with
idf.py on PATH -- no login shell, no alias, no hard-coded path. hil-pool-check documents
that placement so the next rig is set up the same way.