[DX] Add 3 labeled quickstart paths at top of README

Goal

Replace the current feature-first README intro with three labeled, copy-paste quickstart paths covering 90% of first-run scenarios.

What to ship

At the top of the README, add three side-by-side or sequential quickstart blocks:

1. Local quickstart (VS Code, stdio, connection string) — for the "I just want to try it" user. Includes mcp.json snippet, sample prompt, expected response.
2. Copilot CLI quickstart (stdio, connection string) — for the CLI-first user.
3. Hosted deployment quickstart (Container Apps, HTTP, Entra) — for the "we're going to production" user.

Acceptance criteria

• Each block is ≤ 10 lines of config + 1 terminal command + 1 sample prompt.
• All snippets are JSON-correct and tested end-to-end on a clean machine.
• Each block names the persona it serves in the heading (no inference required).
• Links to deeper docs for each, but stands on its own without them.

Why

Today users have to assemble the recipe themselves from a feature list. Three labeled paths cover the common cases and remove decision-paralysis at the door.

Start at the top of README and review the existing feature-first introduction and links to deeper setup documentation. Draft three labeled quickstart blocks for VS Code, Copilot CLI, and hosted Azure Container Apps with the specified snippets, commands, prompts, and persona headings. Validate JSON syntax and test each path end-to-end on a clean machine; done means all acceptance criteria are met and each block stands alone.