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/compaction.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, test } from "bun:test";
+import { getCompactionPrompt } from "../src/compaction/prompt";
+
+describe("compaction prompt", () => {
+  test("returns non-empty string", () => {
+    const prompt = getCompactionPrompt();
+    expect(typeof prompt).toBe("string");
+    expect(prompt.length).toBeGreaterThan(0);
+  });
+
+  test("emphasizes self-continuity, not another agent", () => {
+    const prompt = getCompactionPrompt();
+    expect(prompt).toContain("yourself");
+    expect(prompt).toContain("not another agent");
+  });
+
+  test("includes key sections", () => {
+    const prompt = getCompactionPrompt();
+    expect(prompt).toContain("## Goal");
+    expect(prompt).toContain("## Instructions");
+    expect(prompt).toContain("## Discoveries");
+    expect(prompt).toContain("## Accomplished");
+    expect(prompt).toContain("## Relevant files");
+    expect(prompt).toContain("## Notes");
+  });
+});