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/format.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, test } from "bun:test";
+import { formatMessageList, formatSessionList } from "../src/history/format";
+
+describe("formatSessionList", () => {
+  test("returns message for empty list", () => {
+    expect(formatSessionList([])).toBe("No sessions found.");
+  });
+
+  test("formats sessions as markdown table", () => {
+    const rows = [
+      { id: "ses_abc123def", title: "Test Session", updated: "2024-01-15 10:30:00", msgs: 12 },
+    ];
+    const result = formatSessionList(rows);
+    expect(result).toContain("# Recent Sessions");
+    expect(result).toContain("| ID | Title | Updated | Messages |");
+    expect(result).toContain("ses_abc123de...");
+    expect(result).toContain("Test Session");
+  });
+
+  test("truncates long IDs and titles", () => {
+    const rows = [
+      {
+        id: "ses_verylongidthatshouldbetruncated1234567890",
+        title: "A very long title that should be truncated for display",
+        updated: "2024-01-15",
+        msgs: 5,
+      },
+    ];
+    const result = formatSessionList(rows);
+    expect(result).toContain("ses_verylong...");
+  });
+
+  test("handles untitled sessions", () => {
+    const rows = [{ id: "ses_1", title: null, updated: "2024-01-15", msgs: 0 }];
+    const result = formatSessionList(rows);
+    expect(result).toContain("untitled");
+  });
+});
+
+describe("formatMessageList", () => {
+  test("returns message for empty list", () => {
+    expect(formatMessageList([])).toBe("No messages found.");
+  });
+
+  test("formats messages with roles and timestamps", () => {
+    const rows = [
+      { role: "user", time: "2024-01-15 10:00:00", text: "Hello" },
+      { role: "assistant", time: "2024-01-15 10:00:05", text: "Hi there" },
+    ];
+    const result = formatMessageList(rows);
+    expect(result).toContain("# Conversation");
+    expect(result).toContain("user");
+    expect(result).toContain("assistant");
+    expect(result).toContain("Hello");
+    expect(result).toContain("Hi there");
+  });
+
+  test("truncates long text", () => {
+    const longText = "x".repeat(3000);
+    const rows = [{ role: "assistant", time: "2024-01-15", text: longText }];
+    const result = formatMessageList(rows);
+    expect(result.length).toBeLessThan(longText.length + 200);
+  });
+});