Stop silently installing the devcontainer CLI from the npm registry

[aniongithub]

Problem

install.sh auto-installed the devcontainer CLI by piping the devcontainers/cli standalone installer to sh with all output suppressed (>/dev/null 2>&1). That installer fetches @devcontainers/cli from registry.npmjs.org, which corporate egress policies — e.g. Microsoft Defender's "[TE] NPM URL Block" — intercept.

Because it ran silently:

• The user never saw that a network download was happening.
• When the registry was blocked, the failure was invisible (swallowed output), leaving a confusing broken state.

Fix

Make the devcontainer CLI detect-only, consistent with how devpod and gh are already handled:

• Removed the install_devcontainer_cli() function and the silent curl … | sh pipe.
• The installer now just reports presence:
  • ✓ devcontainer when found, or
  • ✗ devcontainer — https://github.com/devcontainers/cli#install-script (or 'brew install devcontainer') when missing (the brew bottle avoids the npm registry entirely).
• Updated the header comment: "Nothing is downloaded silently on your behalf."

install.ps1 needs no change — it only detects devcontainer in WSL and delegates the binary install to install.sh, so the silent fetch is gone from the Windows/WSL path too. The README already documents manual devcontainer CLI install.

Testing

• bash -n install.sh passes.
• Net change: 8 insertions, 29 deletions (one file).