feat: add curl-based installer and documentation

[Intina47]

Motivation

• Provide an alternative distribution method so users can install Context Sync via a one-line curl | bash flow for environments where invoking npm directly is less convenient.
• Preserve the existing global install behavior and auto-config pathway while making installation easier to discover and run.

Description

• Add a portable bash installer at bin/install.sh that checks for node and npm, supports pinning a release via CONTEXT_SYNC_VERSION, and runs npm install -g "@context-sync/server${VERSION_SUFFIX}".
• Document the new curl install option and system requirements in README.md with the example curl -fsSL https://raw.githubusercontent.com/Intina47/context-sync/main/bin/install.sh | bash command and Node/npm requirement.
• Note the new curl-based installer in docs/RELEASE_NOTES.md and clarify that the curl installer wraps the same npm global install flow.

Testing

• No automated tests were executed for this change.

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Codex Task