Rename knowledge schema to memory; config reads DATABASE_URL

[TheGreatAxios]

Summary

CL-6009 phase 1: renames the package's Postgres schema from knowledge to
memory, and switches config to read DATABASE_URL instead of
KNOWLEDGE_DATABASE_URL.

This is a safe pre-GA rename: no deployment has ever run migrations
against the knowledge schema, so there is no back-compat or migration
concern — the baseline migration SQL is edited in place rather than layered
with a new migration.

• Postgres schema: knowledge → memory (MEMORY_SCHEMA, memorySchema in
  src/db/schema.ts)
• Drizzle table objects renamed for consistency: knowledgeDocument →
  memoryDocument, knowledgeVersion → memoryVersion, knowledgeChunk →
  memoryChunk, knowledgeEntity → memoryEntity, knowledgeEdge →
  memoryEdge, knowledgeEmbedModel → memoryEmbedModel
• arktype schema/type identifiers renamed to match: KnowledgeDocumentSchema
  → MemoryDocumentSchema, KnowledgeVersion* → MemoryVersion*,
  KnowledgeChunk* → MemoryChunk*, KnowledgeEntity* → MemoryEntity*,
  KnowledgeEdge* → MemoryEdge*
• loadMemoryConfig() now reads DATABASE_URL (hard cutover, no fallback to
  the old var name)
• Baseline migration 0002_knowledge_baseline.sql edited in place and
  renamed to 0002_memory_baseline.sql (migrations are discovered/ordered
  by filename prefix, so the rename is safe); 0001_extensions.sql's
  CREATE SCHEMA statement updated too
• Docs (README, AGENTS, PRODUCT, ARCHITECTURE, IMPLEMENTATION, CHANGELOG,
  docs/AUTHZ-DOCUMENT-ACCESS.md), .env.example, and compose.yml updated
  to match
• Prose that uses "knowledge" as a generic domain word (not tied to the
  schema/identifier rename) was left alone

Test plan

• bun run typecheck — passes
• bun test ./src — 320 pass / 0 fail (fully mocked, no live Postgres
  required for this suite)