Improve discoverability: event-sourcing keywords, ecosystem links, and marketplace metadata

[woksin]

What changed

• README.md
  • The opening paragraph now describes Chronicle as the open-source (MIT) event-sourcing database, so the README is findable by the terms people actually search for (event sourcing, event store).
  • "Start here" now links the Chronicle documentation on cratis.io and Lens, Narrator's sibling browser extension for Cratis Arc apps.
  • Added "The Cratis ecosystem" section cross-linking Chronicle and its language clients, Arc, Components, CLI + Workbench, the experimental model-first layer, supporting projects, and Samples.
• Source/VSCodeExtension/package.json (VS Code Marketplace metadata)
  • Richer description: "Browse Cratis Chronicle event stores — event sourcing streams, namespaces, schemas, observers and projections — from VS Code".
  • Added marketplace keywords (event sourcing, event store, cqrs, chronicle, cratis, event-driven) and the "Visualization" category.
  • Added the license field.
• LICENSE (new)
  • Added the MIT license file that the source file headers already reference ("See LICENSE file in the project root").

Why

Part of an organization-wide discoverability pass: make each public Cratis repository accurately describe its place in the stack, use the search terms of its category, and cross-link the rest of the ecosystem. No functional changes.