Move useful reference documents into the Conderis docs #275

Rule

Only documentation that is useful in an open-source context goes into Conderis: architecture, contracts, decisions, how to build and run. The local docs/ folder stays outside git (ignored and removed from history).

Done (2026-09-17)

• item-relations.md → Item relations (#287)
• agent-identity-contract.md → Agent identity and assignees (#288); planning inventory, migration gates and stale owner-label wording dropped, checked against the code
• read-edit-routes.md → Web routes (#289), extended with the documentation routes
• project-item-references.md → merged into Item numbering (#284); the typed diagnostic form was never implemented and is left out
• privacy-deletion.md → already covered by Deleting content (#286)

Deliberately not moved

• security-review-2026-09-13.md and its probes: internal security details
• memory-freshness-review-2026-09-13.md, memory-maintenance-plan-2026-09-13.json: internal maintenance notes about stored memories
• proposal-migration.md: historical migration note without lasting value
• Operations and hosting notes: not development knowledge for a public project