feat request: Document secure runtime injection for OPENAI_API_KEY via password manager CLI

Problem

Right now the practical choices for OPENAI_API_KEY are mostly:

• store it directly in ~/.config/shell_gpt/.sgptrc
• export it in the shell environment before running sgpt

Both are awkward from a security perspective:

• plaintext in .sgptrc is easy to leak accidentally
• globally exporting the key makes it available to unrelated subprocesses in that shell session

Narrow request

I am not asking for a full secret-management subsystem here. A smaller improvement could be:

• document password-manager-based runtime injection as a recommended pattern
• or add a minimal first-class mechanism for loading sensitive values at runtime

Current workaround

I am using pass and a shell wrapper so the key is injected only for the sgpt process:

if tty -s; then
export GPG_TTY="$(tty)"
fi

sgpt() {
local key
key="$(pass show api/openrouter/sgpt | head -n1)" || return
OPENAI_API_KEY="$key" command sgpt "$@"
}

This keeps secrets out of .sgptrc and avoids exporting them to the whole shell session.

In my case I originally tried putting command substitution directly into .sgptrc, but ShellGPT reads that file literally, so something like:

OPENAI_API_KEY=$(pass show api/openrouter/sgpt | head -n1)

does not work.

Related existing issues

#709, #588