docs: add complete getting started guide with token saving walkthrough

Author: manojmallick

README.md

@@ -48,6 +48,8 @@
 | [Project structure](#-project-structure) | File-by-file map |
 | [Principles](#-principles) | Design decisions |
 
+> 📖 **New to SigMap?** Read the **[Complete Getting Started Guide](docs/GETTING_STARTED.md)** — token savings walkthrough, every command, VS Code plugin, and CI setup.
+
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 ## 🔍 What it does