Revert "feat(knowledgebase): replace litellm-pgvector connector with memory-retrieval"

This reverts commit abeadc49c8.

Restores vendor/litellm-pgvector/ and the vector_store_registry wiring
(in-band file_search tool-call support) at the user's request, after
re-confirming against docs.litellm.ai/docs/completion/knowledgebase and
litellm-pgvector's own README that pg_vector is still not an in-process
vector_store_registry backend -- it requires this same standalone
connector service either way, so there is no simpler 'native' path that
was missed. Trading back in: 793 lines of vendored code, the untested
Prisma migration, and the git-context build risk noted in VENDORED.md
(all flagged as unverified against real hardware in issue #24), in
exchange for the file_search in-band tool call memory-retrieval did not
support.

Conflicts resolved on top of later commits (Redis, update.sh key-minting
fold-in):
- .env.example / docs/memory-knowledgebase.md: kept the auto-mint-via-
  update.sh language, renamed MEMORY_RETRIEVAL_* back to
  LITELLM_PGVECTOR_*.
- scripts/generate-secrets.sh: left deleted -- its job was folded into
  update.sh in 24d749b, unrelated to this revert.
- scripts/update.sh: renamed the MEMORY_RETRIEVAL_* secret/mint calls to
  LITELLM_PGVECTOR_* to match.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018WHfjWrSEcGhCoeu6dQfDa

vendor/litellm-pgvector/prisma/schema.prisma

@@ -0,0 +1,40 @@
+// This is your Prisma schema file,
+// learn more about it in the docs: https://pris.ly/d/prisma-schema
+
+generator client {
+  provider = "prisma-client-py"
+}
+
+datasource db {
+  provider = "postgresql"
+  url      = env("DATABASE_URL")
+}
+
+model VectorStore {
+  id           String   @id @default(cuid())
+  name         String
+  file_counts  Json?
+  status       String   @default("completed")
+  usage_bytes  Int?     @default(0)
+  created_at   DateTime @default(now())
+  expires_after Json?
+  expires_at   DateTime?
+  last_active_at DateTime?
+  metadata     Json?
+  embeddings   Embedding[]
+  
+  @@map("vector_stores")
+}
+
+model Embedding {
+  id              String      @id @default(cuid())
+  vector_store_id String
+  content         String
+  embedding       Unsupported("vector(1536)")
+  metadata        Json?
+  created_at      DateTime    @default(now())
+  
+  vector_store    VectorStore @relation(fields: [vector_store_id], references: [id], onDelete: Cascade)
+  
+  @@map("embeddings")
+}