docs: overhaul README with badges, features and how-it-works sections

[mpicciolli]

Summary

Reworks the README into a more polished, npm-package-style landing page while keeping all technical details accurate.

• Adds a centered header with npm version, license, node version and MCP-compatible badges
• Introduces a Features section summarizing the server's capabilities
• Adds a How it works section explaining the stateless, read-only, in-memory SQLite model
• Consolidates the client install instructions (Claude/ChatGPT/Gemini via npx) into collapsible sections
• Expands the Development section to list all available npm scripts (test:watch, coverage, pack, MCP Inspector)
• Uses GitHub [!IMPORTANT] / [!NOTE] admonitions for the read-only guarantee and bundle auto-update caveat

All claims verified against package.json: Node >=22.0.0 and every referenced script (build, test, test:watch, coverage, lint, format, pack, inspector) exist.

[Copilot]

Pull request overview

This PR refreshes README.md into a more polished, npm-style landing page for the pcm-mcp MCP server, emphasizing core capabilities, safety guarantees, and clearer onboarding for multiple clients.

Changes:

• Adds a centered header section with badges and a short product-style intro.
• Introduces/expands Features, Getting started, and How it works sections to better explain capabilities and the stateless read-only architecture.
• Consolidates installation instructions into collapsible sections and expands the Development section with commonly used scripts and MCP Inspector guidance.

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