Prepare CORNETO 1.0.0 RC6

[pablormier]

Summary

• Prepare the CORNETO 1.0.0-rc.6 release notes and add them to the release index.
• Promote the already-public dev snapshot to main, including the new LinearDAGDiscovery, expanded CellNOptDAG workflows, dataset helpers, and AnnNet compatibility updates.
• Remove an extra blank line and fix the LinearDAG guide test to use the guide's current docs/guide/causal/ location.

Validation

• git diff --check public/main...HEAD passes.
• Focused causal, CellNOptDAG, AnnNet, and predictive API tests: 80 passed, 71 skipped in the available environment.
• The notebook execution test passes when run separately with permission for its local Jupyter kernel to bind a port.
• AnnNet runtime tests were skipped because the optional AnnNet dependency is unavailable in the local environment.