Add JSON ingestion gem shell

[jwils]

Why

Split the JSON ingestion extraction into reviewable steps so the gem packaging can be checked independently before code moves.

What

• Add the elasticgraph-json_ingestion gem shell
• Wire the new path gem into the bundle without changing runtime behavior
• Update generated dependency diagrams and gem READMEs for the new empty gem

Risk Assessment

Low — this only adds an empty extension gem package, lockfile wiring, and generated dependency docs.

References

• First PR in the JSON ingestion extraction stack.
• Local spellcheck, lint, type-check, schema artifact, and dependency diagram checks passed.