[Lane 2 · kotlin-core] NotImplemented stubs — Markdown output for the operator docs generator #1418

Sub-issue of #1414 ([Feature]: Close the remaining NotImplemented stubs outside the operator surface).

Lane: 2 · Kotlin core
Skill needed: Kotlin (Gradle build-logic). No SKaiNET runtime knowledge needed.
Size: s (a few hours)
Blocked by: nothing

What to do

1. GenerateDocumentationTask (build-logic/convention/src/main/kotlin/GenerateDocumentationTask.kt) throws for DocumentationFormat.MARKDOWN (and HTML).
2. Implement Markdown by mirroring generateAsciidoc (index, one page per operator, coverage matrix as a Markdown table).
3. Lower the file's count in config/stub-allowlist.txt from 2 to 1 (HTML stays); run ./gradlew checkUnmarkedStubs and try it with documentation { format.set(DocumentationFormat.MARKDOWN) } locally.

Acceptance

• Markdown output renders the same rows as the AsciiDoc matrix
• Result reported back on the parent issue

Notes

If something here turns out to be wrong or much bigger than the size label, comment on this issue or open a specific bug instead of blocking the parent.

Coverage matrix: docs/modules/ROOT/pages/reference/ops-status-matrix.adoc · How lanes work: Issue taxonomy