fix: give pnpm installer a default SHELL to avoid ERR_PNPM_UNKNOWN_SHELL

[bushidocodes]

Problem

The pnpm install recipe pipes the installer to sh -, and the installer infers which rc file to update from $SHELL. When $SHELL is unset (non-interactive shells, some CI/cron/container contexts) it aborts:

[ERR_PNPM_UNKNOWN_SHELL] Could not infer shell type.
Set the SHELL environment variable to your active shell.
Install Error!  ->  make: *** [Makefile: .../bin/pnpm] Error 1

Fix

Default SHELL to /bin/bash for the installer when it isn't already set (a normal interactive run keeps using the real $SHELL):

curl -fsSL https://get.pnpm.io/install.sh | SHELL="$${SHELL:-/bin/bash}" sh -

Verification

In a fresh ubuntu:24.04 container with SHELL explicitly unset:

# old: curl ... | sh -
[ERR_PNPM_UNKNOWN_SHELL] Could not infer shell type.  Install Error!

# new: curl ... | SHELL="${SHELL:-/bin/bash}" sh -
Done in 3.8s using pnpm v11.7.0   (binary installed)

Fixes #49

🤖 Generated with Claude Code