feat: inject conversation history into session work model

- Add session_messages table and migration for storing conversation turns
- Add getSessionHistory() helper to load recent history with token budget
- Pass conversation history to executeWork() and executeWorkStreaming()
- Persist user/assistant exchanges after completed requests
- Rejected/failed requests do not pollute history

Fixes #39

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/db/migrations/0008_session_messages.sql

@@ -0,0 +1,13 @@
+-- Migration: Session conversation history (#38/#39)
+-- Stores user/assistant message pairs for context injection into the work model.
+
+CREATE TABLE IF NOT EXISTS session_messages (
+  id          SERIAL PRIMARY KEY,
+  session_id  TEXT NOT NULL REFERENCES sessions(id),
+  role        TEXT NOT NULL,
+  content     TEXT NOT NULL,
+  token_count INTEGER,
+  created_at  TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX idx_session_messages_session_id ON session_messages(session_id);

lib/db/src/index.ts

@@ -1,4 +1,5 @@
 import { drizzle } from "drizzle-orm/node-postgres";
+import { eq, asc } from "drizzle-orm";
 import pg from "pg";
 import * as schema from "./schema";
 
@@ -14,3 +15,46 @@ export const pool = new Pool({ connectionString: process.env.DATABASE_URL });
 export const db = drizzle(pool, { schema });
 
 export * from "./schema";
+
+// ── Session history helper ──────────────────────────────────────────────────
+
+/**
+ * Load the most recent conversation history for a session, capped by turn
+ * count and approximate token budget.
+ *
+ * @param sessionId  Session to load history for
+ * @param maxTurns   Maximum number of messages to return (default 8)
+ * @param maxTokens  Approximate token budget — stops including older messages
+ *                   once cumulative token_count exceeds this (default 4000)
+ * @returns Array of { role, content } objects in chronological order
+ */
+export async function getSessionHistory(
+  sessionId: string,
+  maxTurns = 8,
+  maxTokens = 4000,
+): Promise<Array<{ role: "user" | "assistant"; content: string }>> {
+  const rows = await db
+    .select({
+      role: schema.sessionMessages.role,
+      content: schema.sessionMessages.content,
+      tokenCount: schema.sessionMessages.tokenCount,
+    })
+    .from(schema.sessionMessages)
+    .where(eq(schema.sessionMessages.sessionId, sessionId))
+    .orderBy(asc(schema.sessionMessages.id));
+
+  // Take the most recent messages that fit within budget
+  const result: Array<{ role: "user" | "assistant"; content: string }> = [];
+  let totalTokens = 0;
+
+  // Walk from newest to oldest, then reverse
+  for (let i = rows.length - 1; i >= 0 && result.length < maxTurns; i--) {
+    const row = rows[i]!;
+    const tokens = row.tokenCount ?? Math.ceil(row.content.length / 4);
+    if (totalTokens + tokens > maxTokens && result.length > 0) break;
+    totalTokens += tokens;
+    result.push({ role: row.role as "user" | "assistant", content: row.content });
+  }
+
+  return result.reverse();
+}

lib/db/src/schema/index.ts

@@ -13,3 +13,4 @@ export * from "./nostr-trust-vouches";
 export * from "./relay-accounts";
 export * from "./relay-event-queue";
 export * from "./job-debates";
+export * from "./session-messages";

lib/db/src/schema/session-messages.ts

@@ -0,0 +1,18 @@
+import { pgTable, text, timestamp, integer, serial } from "drizzle-orm/pg-core";
+import { sessions } from "./sessions";
+
+// ── session_messages ────────────────────────────────────────────────────────
+// Stores conversation history for context injection into the work model.
+
+export const sessionMessages = pgTable("session_messages", {
+  id: serial("id").primaryKey(),
+  sessionId: text("session_id")
+    .notNull()
+    .references(() => sessions.id),
+  role: text("role").$type<"user" | "assistant">().notNull(),
+  content: text("content").notNull(),
+  tokenCount: integer("token_count"),
+  createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
+});
+
+export type SessionMessage = typeof sessionMessages.$inferSelect;