Replace sqlite3 subprocess with bun:sqlite native driver, add AGENTS.md and tests

Rewrites history/queries.ts to use bun:sqlite with readonly mode and
parameterized queries instead of spawning sqlite3. Consolidates threshold
constants into a single source of truth. Adds AGENTS.md as the canonical
operational doc, moves architecture to docs/, and adds initial test suite.

tests/queries.test.ts

@@ -0,0 +1,76 @@
+import { Database } from "bun:sqlite";
+import { describe, expect, test } from "bun:test";
+import { mkdirSync, mkdtempSync, rmSync } from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { closeDb, runQuery } from "../src/history/queries";
+
+describe("queries module with bun:sqlite", () => {
+  const tmpDir = mkdtempSync(path.join(os.tmpdir(), "open-memory-test-"));
+  const opencodeDir = path.join(tmpDir, "opencode");
+  const dbPath = path.join(opencodeDir, "opencode.db");
+  const originalXdg = process.env.XDG_DATA_HOME;
+
+  const setup = (): void => {
+    mkdirSync(opencodeDir, { recursive: true });
+    const db = new Database(dbPath);
+    db.run(
+      "CREATE TABLE IF NOT EXISTS project (id TEXT PRIMARY KEY, worktree TEXT, name TEXT, time_updated INTEGER)",
+    );
+    db.run(
+      "CREATE TABLE IF NOT EXISTS session (id TEXT, project_id TEXT, parent_id TEXT, title TEXT, summary TEXT, time_created INTEGER, time_updated INTEGER)",
+    );
+    db.run(
+      "INSERT OR REPLACE INTO project VALUES ('proj1', '/tmp/test', 'test-project', 1700000000000)",
+    );
+    db.run(
+      "INSERT OR REPLACE INTO session VALUES ('ses_1', 'proj1', NULL, 'Test Session', NULL, 1700000000000, 1700001000000)",
+    );
+    db.close();
+    process.env.XDG_DATA_HOME = opencodeDir;
+  };
+
+  const cleanup = (): void => {
+    closeDb();
+    process.env.XDG_DATA_HOME = originalXdg;
+    rmSync(tmpDir, { recursive: true, force: true });
+  };
+
+  test("runQuery without parameters", () => {
+    setup();
+    try {
+      type CountRow = { label: string; cnt: number };
+      const rows = runQuery<CountRow>("SELECT 'projects' AS label, COUNT(*) AS cnt FROM project");
+      expect(rows.length).toBe(1);
+      expect(rows[0].cnt).toBe(1);
+    } finally {
+      cleanup();
+    }
+  });
+
+  test("runQuery with named parameters", () => {
+    setup();
+    try {
+      type SessionRow = { id: string; title: string };
+      const rows = runQuery<SessionRow>("SELECT id, title FROM session WHERE id = $sessionId", {
+        $sessionId: "ses_1",
+      });
+      expect(rows.length).toBe(1);
+      expect(rows[0].id).toBe("ses_1");
+      expect(rows[0].title).toBe("Test Session");
+    } finally {
+      cleanup();
+    }
+  });
+
+  test("runQuery opens database read-only", () => {
+    setup();
+    try {
+      type Row = { cnt: number };
+      const rows = runQuery<Row>("SELECT COUNT(*) AS cnt FROM session");
+      expect(rows[0].cnt).toBe(1);
+    } finally {
+      cleanup();
+    }
+  });
+});