Add guided Keymaster installer

[macterra]

Summary

• add a guided install script for Keymaster onboarding
• prompt for ID name, passphrase, and Gatekeeper URL after checking Node.js and npm
• document the curl-to-bash installer flow and env var persistence guidance

Closes #253

[Copilot]

Pull request overview

Adds a guided “curl | bash” onboarding flow for installing and configuring the Keymaster CLI, plus documentation updates describing how the installer works and how to persist required environment variables.

Changes:

• Added scripts/install-keymaster.sh interactive installer (Node/npm detection, Keymaster install, prompts for ID/passphrase/Gatekeeper URL, runs keymaster create-id).
• Documented the hosted installer command in the root README and the @didcid/keymaster package README.
• Added guidance about environment variable persistence when running via curl | bash.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
scripts/install-keymaster.sh	New guided installer script for Keymaster onboarding and initial ID creation.
packages/keymaster/README.md	Documents the guided installer and explains the prompt/persistence behavior.
README.md	Adds a “Keymaster Onboarding” section pointing users to the hosted installer flow.

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