fix: use shell env var ${RUNNER_TEMP} in run blocks, ${{ runner.temp }} in with/env fields

Fixes CI failure caused by smoke-claude.md exceeding GitHub Actions'
21KB expression size limit. The longer ${{ runner.temp }} expression
(23 chars vs /opt/gh-aw's 10) pushed run: blocks over the limit.

Solution: use ${RUNNER_TEMP} (shell env var, 16 chars) in run: blocks
where shell resolves the variable, and keep ${{ runner.temp }} only
in with:/env: fields where GitHub Actions template resolution is needed.

Also adds GhAwRootDirShell constant and SetupActionDestinationShell
for the two path forms.

Co-authored-by: Mossaka <5447827+Mossaka@users.noreply.github.com>

Author: Copilot