Surface the simplest "run a migration" command in the README, with pointers to deeper docs

## Problem

A new user couldn't find the basic how-to in the repo and ended up relying on the blog and a slide deck instead, then asked why that material wasn't in the repo itself. The minimal run command lives in `docs/command-contract.md`, which they found overly formal, and it isn't surfaced prominently from the README.

## User impact

The fastest path to value — "migrate one dashboard" — is effectively hidden, so new users bounce to external material or stall before their first successful run.

## Proposed outcome

Add a short "Run your first migration" section to the README, right after Quick Start:

- One copy-paste, file-based Grafana example (the simplest happy path).
- Clear pointers to where to go deeper: the full command reference (`docs/command-contract.md`), where run outputs/artifacts land and how to read them (the generated `yaml/` and `compiled/` directories plus the verification summary), and known limitations (`docs/known-limitations.md`).
- Frame `command-contract.md` as the reference, not the starting point.

## Success criteria

A new user can migrate one dashboard from the README alone, and knows from the README where to find run output and where to read more.

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*Source: user feedback during first-run onboarding.*

Start with the README's Quick Start section and read docs/command-contract.md to identify the simplest file-based Grafana migration command. Check docs/known-limitations.md and the references to generated yaml/ and compiled/ directories and the verification summary. Done means a new user can migrate one dashboard from the README and can find the command reference, output guidance, and limitations.

Written by the indexing model from the issue text.