docs: Claude skills (#410)

* docs: add CLAUDE.md with build commands and architecture guidance

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Eric Deandrea <eric.deandrea@ibm.com>

* docs: add DCO sign-off convention to CLAUDE.md

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Eric Deandrea <eric.deandrea@ibm.com>

---------

Signed-off-by: Eric Deandrea <eric.deandrea@ibm.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: edeandrea

CLAUDE.md

@@ -61,5 +61,6 @@ This is a multi-module Gradle project (Kotlin DSL) with group `ai.docling`. Vers
 ## Conventions
 
 - Commits must follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+- All commits must include DCO sign-off (`git commit -s`)
 - `spotless` uses `ratchetFrom("origin/main")` — only files changed relative to `origin/main` are checked
 - Line endings: LF; no trailing whitespace; files must end with a newline (`.editorconfig`)