No API reference or user guide beyond the README (implementation-plan Task 7.2 unfinished) #163

Plan task

docs/implementation-plan.md, Task 7.2 "Comprehensive Documentation":

• Generate XML documentation
• Create detailed user guide with examples
• Add API reference documentation

The same commitment appears in docs/design.md ("Advanced documentation and API reference", ~L238-239).

What exists today

• XML documentation is generated for the Coder assembly via ktsu.Sdk (ktsu.Coder.xml in the build output). That bullet is done.
• README.md has a Quick Start and per-feature sections.
• docs/ contains only design.md and implementation-plan.md.

What's missing

• A published or committed API reference generated from the XML docs (for example docfx, or generated markdown under docs/api/).
• A user guide that walks through building an AST, choosing generators, YAML round-tripping and the editor, beyond the README's snippets.

The per-language samples/example-repository part is already tracked separately as #139 (Task 7.3) and is out of scope here.

Acceptance criteria

• The API reference is generated from the existing XML docs and either committed under docs/ or produced as a CI artifact, and linked from the README.
• A user guide under docs/ covers: building an AST, generating code for each supported language, saving and loading YAML, and using Coder.Editor.

Dependencies

None. Can proceed independently of #139.