feat(agents): add Agent + ChatThread + ChatMessage schema (Stage 1)

Introduces the persistence layer for the upcoming Agent feature: an LLM
persona pinned to a specific Llm, optionally attached to a Project, with
persisted chat threads/messages so conversations survive REPL exits.

Constraint shape:
- Agent.llm uses ON DELETE RESTRICT — deleting an Llm in active use fails.
- Agent.project uses ON DELETE SET NULL — agents survive project deletion.
- ChatThread → ChatMessage cascade so deleting an agent purges its history.
- ChatMessage @@unique([threadId, turnIndex]) gives append ordering even
  under racing writers (services retry on collision).

LiteLLM-style per-call overrides will live in Agent.defaultParams (Json);
the loose extras Json field is reserved for future LoRA/tool-allowlist work.

Pinned vitest fileParallelism=false in @mcpctl/db: all suites share the
same Postgres, and adding a second suite exposed FK contention between a
clearAllTables in one file and a create in another. Per-test isolation
still comes from beforeEach.

Tests: 8/8 green in src/db/tests/agent-schema.test.ts (defaults, name
uniqueness, llm-in-use Restrict, project-delete SetNull, agent-delete
cascade, duplicate (threadId, turnIndex) blocked, tool-call payload
round-trip, lastTurnAt DESC ordering).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/db/prisma/migrations/20260425160000_add_agents_and_chat/migration.sql

@@ -0,0 +1,91 @@
+-- CreateTable
+CREATE TABLE "Agent" (
+    "id" TEXT NOT NULL,
+    "name" TEXT NOT NULL,
+    "description" TEXT NOT NULL DEFAULT '',
+    "systemPrompt" TEXT NOT NULL DEFAULT '',
+    "llmId" TEXT NOT NULL,
+    "projectId" TEXT,
+    "proxyModelName" TEXT,
+    "defaultParams" JSONB NOT NULL DEFAULT '{}',
+    "extras" JSONB NOT NULL DEFAULT '{}',
+    "ownerId" TEXT NOT NULL,
+    "version" INTEGER NOT NULL DEFAULT 1,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "Agent_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "ChatThread" (
+    "id" TEXT NOT NULL,
+    "agentId" TEXT NOT NULL,
+    "ownerId" TEXT NOT NULL,
+    "title" TEXT NOT NULL DEFAULT '',
+    "lastTurnAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "updatedAt" TIMESTAMP(3) NOT NULL,
+
+    CONSTRAINT "ChatThread_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateTable
+CREATE TABLE "ChatMessage" (
+    "id" TEXT NOT NULL,
+    "threadId" TEXT NOT NULL,
+    "turnIndex" INTEGER NOT NULL,
+    "role" TEXT NOT NULL,
+    "content" TEXT NOT NULL,
+    "toolCalls" JSONB,
+    "toolCallId" TEXT,
+    "status" TEXT NOT NULL DEFAULT 'complete',
+    "createdAt" TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "ChatMessage_pkey" PRIMARY KEY ("id")
+);
+
+-- CreateIndex
+CREATE UNIQUE INDEX "Agent_name_key" ON "Agent"("name");
+
+-- CreateIndex
+CREATE INDEX "Agent_name_idx" ON "Agent"("name");
+
+-- CreateIndex
+CREATE INDEX "Agent_llmId_idx" ON "Agent"("llmId");
+
+-- CreateIndex
+CREATE INDEX "Agent_projectId_idx" ON "Agent"("projectId");
+
+-- CreateIndex
+CREATE INDEX "Agent_ownerId_idx" ON "Agent"("ownerId");
+
+-- CreateIndex
+CREATE INDEX "ChatThread_agentId_lastTurnAt_idx" ON "ChatThread"("agentId", "lastTurnAt" DESC);
+
+-- CreateIndex
+CREATE INDEX "ChatThread_ownerId_idx" ON "ChatThread"("ownerId");
+
+-- CreateIndex
+CREATE INDEX "ChatMessage_threadId_createdAt_idx" ON "ChatMessage"("threadId", "createdAt");
+
+-- CreateIndex
+CREATE UNIQUE INDEX "ChatMessage_threadId_turnIndex_key" ON "ChatMessage"("threadId", "turnIndex");
+
+-- AddForeignKey
+ALTER TABLE "Agent" ADD CONSTRAINT "Agent_llmId_fkey" FOREIGN KEY ("llmId") REFERENCES "Llm"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "Agent" ADD CONSTRAINT "Agent_projectId_fkey" FOREIGN KEY ("projectId") REFERENCES "Project"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "Agent" ADD CONSTRAINT "Agent_ownerId_fkey" FOREIGN KEY ("ownerId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ChatThread" ADD CONSTRAINT "ChatThread_agentId_fkey" FOREIGN KEY ("agentId") REFERENCES "Agent"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ChatThread" ADD CONSTRAINT "ChatThread_ownerId_fkey" FOREIGN KEY ("ownerId") REFERENCES "User"("id") ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "ChatMessage" ADD CONSTRAINT "ChatMessage_threadId_fkey" FOREIGN KEY ("threadId") REFERENCES "ChatThread"("id") ON DELETE CASCADE ON UPDATE CASCADE;