docs: align memory discovery references

[yaacovcorcos]

Summary

• replace one stale checklist reference to the Idea Inbox with the draft Memory Architecture Discovery
• describe the discovery questions as open rather than unprocessed
• preserve the explicit non-authorization boundary for memory implementation

Validation

• git diff --check
• node --test lab/scripts/verify-owned-sources.test.mjs
• node lab/scripts/verify-owned-sources.mjs
• node lab/scripts/verify-gate-1-5-manifest.mjs
• repository search confirms no stale memory-specific Idea Inbox references remain