Add live documentation (DocFX) and complete C4 architecture diagrams

[Copilot]

The project had no documentation beyond a sparse README with a :TODO placeholder for API docs and no architectural reference.

Documentation site (DocFX)

• docfx.json — DocFX config; generates a full browsable site (API reference extracted from XML comments + Markdown pages) via:

  dotnet tool install -g docfx
  docfx docfx.json --serve   # → http://localhost:8080

• docs/toc.yml — navigation table-of-contents for DocFX

Docs pages

• docs/index.md — landing page: overview, quick-start snippet, feature list
• docs/getting-started.md — install → load scripts → sortReplies() → reply(); covers topics, object macros, custom ISessionManager, error handling, full Config reference
• docs/api/index.md — API reference index for every public type, grouped by subsystem

C4 architecture (all four levels, Mermaid diagrams)

File	Level
docs/architecture/c4-context.md	L1 System Context — developer, host app, scripts, optional session store
docs/architecture/c4-container.md	L2 Container — engine, parser, topic manager, session manager, object handler, sorting
docs/architecture/c4-component.md	L3 Component — all key classes per subsystem with responsibility tables
docs/architecture/c4-code.md	L4 Code — classDiagram blocks for Engine, Parser+AST, TopicManager+Sorting, Session, ObjectHandlers, Logging

README

• Replaced :TODO docs section with links to the new docs and DocFX serve instructions
• Marked documentation item done in the task list

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