[codex] Add product end-of-life feature to plan

[OneTwo3D]

Summary

• Adds P8.1 to docs/plan.md for product End of Life lifecycle controls.
• Covers product-page checkbox, reorder forecast exclusion, filters for EOL products, and automatic archival when all stock and incoming supply are exhausted.
• Adds concrete acceptance criteria and tests for UI persistence, forecasts, filters, auto-archive blocking conditions, and activity logging.

Validation

• git diff --check: pass
• No code validation run; documentation-only plan update.

Beads

• Closed onetwo3d-ims-5z1 and exported .beads/issues.jsonl.

[OneTwo3D]

Superseded by PR #156 ("Implement product lifecycle and supplier reorder planning", merge commit 3581d65 on development). P8.1 in docs/plan.md is marked Complete and the product End-of-Life lifecycle (DRAFT/ACTIVE/EOL/ARCHIVED), reorder forecast exclusion, EOL filter, and auto-archive cron all shipped. Safe to discard.