Improve discoverability: event-sourcing positioning, ecosystem links, and package keywords - #166

What changed

• README.md — ties the language explicitly to event-sourced, CQRS systems (commands, events, projections) and labels it as part of the experimental Cratis model-first layer; hyperlinks Stage, Studio, Chronicle, and Arc on first mention; adds a "The Cratis ecosystem" section cross-linking Chronicle (and its language clients), Arc, Components, CLI + Workbench, the model-first layer, supporting projects, and Samples.
• Documentation/index.md — adds frontmatter title/description with event-sourcing and event-modeling wording, matching the pattern used by sibling model-first repositories.
• Source/DotNET/Screenplay/Screenplay.csproj and Source/DotNET/Tool/Tool.csproj — add NuGet PackageTags (event-sourcing, event-modeling, cqrs, dsl, compiler; dotnet-tool for the CLI).
• Source/Screenplay/Monaco/screenplay-language/package.json and Source/Screenplay/VSCodeExtension/package.json — add keywords arrays so the npm package and the VS Code Marketplace listing surface in event-sourcing / CQRS / DSL searches.

Note: the README's emoji headings are valid UTF-8 (verified byte-level) — no glyph fixes were needed.

Why

Screenplay never mentioned the category terms people actually search for (event sourcing, CQRS, event modeling), and its packages shipped without tags or keywords. This aligns the repository with the wording used across the Cratis organization: accurate breadth, consistent ecosystem cross-links, and category keywords on every published surface — no functional changes.