feat(create-slack-app): read API keys from shell env before prompting

[mwbrooks]

Summary

This pull request lets the create-slack-app skill pick up the AI provider API key from the developer's shell environment instead of always prompting for it in chat.

• Step 4c now probes for the required env var (ANTHROPIC_API_KEY, OPENAI_API_KEY) with a non-echoing check before invoking AskUserQuestion.
• When the variable is set, the value is passed through to slack env set by reference ("$ANTHROPIC_API_KEY"), so Bash expands it at run time and the literal key never lands in the chat transcript.
• When the variable is missing, the existing AskUserQuestion prompt still runs — with a one-line note telling the developer they can export the variable ahead of time to skip the prompt in future sessions.
• Tightens the "do not echo" guard to also forbid echo $VAR, printenv, and slack env list, all of which would print the stored value.

Motivated by demo recordings and screen-shared sessions where the existing flow would render an API key in plaintext in the visible transcript.

Testing

• With ANTHROPIC_API_KEY exported in the shell, run create-slack-app against an agent template (e.g. bolt-python-starter-agent with the claude-agent-sdk subdir) and confirm:
  • The skill reports the key was picked up from the environment.
  • No AskUserQuestion prompt fires for the key.
  • The value never appears in the chat transcript.
  • The app launches and the agent template runs.
• With the env var unset, run the same flow and confirm:
  • The AskUserQuestion prompt still fires.
  • The note about exporting the variable in future sessions is shown.
  • slack env set succeeds with the pasted value.