docs: add memory architecture discovery

[yaacovcorcos]

Summary

• move the future Scient memory discussion from the lightweight idea inbox into a dedicated draft discovery document
• preserve candidate scopes, vocabulary, trust and lifecycle questions, local/cloud/Git constraints, scenarios, and a technology-neutral discovery sequence
• remove the triaged memory entry from the inbox and update planning, architecture, and source-trace references
• keep the obsolete persistence-decision brief absent and make clear that no database, schema, sync protocol, or memory implementation is selected

This is a documentation-only follow-up to merged parent PR #23 and merged desktop PR #14. It does not reopen or block the completed T3 reliability work.

Validation

• node lab/scripts/verify-gate-1-5-manifest.mjs
• node --test lab/scripts/verify-owned-sources.test.mjs
• node lab/scripts/verify-owned-sources.mjs
• changed-document relative-link validation
• git diff --check
• confirmed docs/architecture/scient-project-persistence-decision-brief.md is absent and has no active references