Add end-to-end epics proposal

[shchukins]

Summary

• Adds docs/product/END_TO_END_EPICS_PROPOSAL.md
• Proposes six scenario-oriented product epics based on repo docs and current GitHub issues
• Maps existing issues into end-to-end loops and recommends renames, merges, splits, closures, and missing issues

Notes

• This PR does not create, close, rename, relabel, or otherwise edit GitHub issues.
• gh was not installed locally, so issue inspection used the GitHub connector instead.

Verification

• Documentation-only change; no tests run.