fix(dashboard): remove immutable memory controls

[mameikagou]

Summary

• remove the Decay page Boost action because rewrite memories reject PATCH mutations
• remove Chat memory reinforcement controls because the compatibility proxy returns an immutable-memory conflict
• describe cited memories as read-only, evidence-driven references in the chat setup guide
• add regression coverage so unsupported mutation controls are not reintroduced

The ingest and query flows remain available; this only removes controls that cannot succeed against the rewrite API.

Validation

• node --test dashboard/tests/mutation-controls.test.js (2 passed)
• cd dashboard && node --test tests/mutation-controls.test.js (2 passed)
• cd dashboard && npm run build
• git diff --check

npm run lint is not available in this branch because ESLint is not declared or installed.