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/thresholds.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test } from "bun:test";
+import { type ContextStatus, getStatusLabel, THRESHOLDS } from "../src/context/thresholds";
+
+describe("thresholds", () => {
+  test("THRESHOLDS constants", () => {
+    expect(THRESHOLDS.yellow).toBe(70);
+    expect(THRESHOLDS.red).toBe(85);
+    expect(THRESHOLDS.critical).toBe(92);
+  });
+
+  test("getStatusLabel returns correct status", () => {
+    expect(getStatusLabel(0)).toBe("green");
+    expect(getStatusLabel(50)).toBe("green");
+    expect(getStatusLabel(69)).toBe("green");
+    expect(getStatusLabel(70)).toBe("yellow");
+    expect(getStatusLabel(75)).toBe("yellow");
+    expect(getStatusLabel(84)).toBe("yellow");
+    expect(getStatusLabel(85)).toBe("red");
+    expect(getStatusLabel(90)).toBe("red");
+    expect(getStatusLabel(91)).toBe("red");
+    expect(getStatusLabel(92)).toBe("critical");
+    expect(getStatusLabel(99)).toBe("critical");
+    expect(getStatusLabel(100)).toBe("critical");
+  });
+
+  test("ContextStatus type accepts all valid values", () => {
+    const statuses: ContextStatus[] = ["green", "yellow", "red", "critical"];
+    for (const s of statuses) {
+      expect(typeof s).toBe("string");
+    }
+  });
+});