_pip_install_fallback_chain silently discarded pip stderr via
2>/dev/null on every attempt. When pip failed (network error, venv
mismatch, disk full), the wrapper exited 0 and the Cookbook UI showed
the download as running — the silent-failure mode from #354.
Extract _pip_install_attempt() which wraps each pip invocation in a
bash -c subshell that captures output to a temp file, prints tail -5
on failure, cleans up, and exits with pip's real exit code. This
avoids the | tail pipefail masking (the first blocker on #363) while
surfacing the last 5 lines of pip output in the tmux log so users
can see what went wrong.
Both local wrapper and remote SSH runner use the same helper through
_pip_install_fallback_chain, so the fix is symmetric.
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