Add unit tests for confidence scoring module

[manthan787]

Description

Improve test coverage for the confidence scoring module in src/agent/confidence.ts.

Why

The confidence scoring module is critical for investigation quality but currently has limited test coverage. Adding tests helps ensure reliability and makes future changes safer.

What to do

1. Look at src/agent/confidence.ts to understand the confidence calculation logic
2. Create or extend src/agent/__tests__/confidence.test.ts
3. Add test cases for:
   • Edge cases (0%, 100% confidence)
   • Multiple evidence sources
   • Conflicting evidence handling
   • Confidence threshold behavior

Example test structure

import { describe, test, expect } from "bun:test";
import { calculateConfidence } from "../confidence";

describe("confidence scoring", () => {
  test("returns 0 with no evidence", () => {
    // ...
  });
  
  test("increases confidence with supporting evidence", () => {
    // ...
  });
});

Getting Started

bun install
bun test src/agent/__tests__/confidence.test.ts

Great for learning the testing patterns in this project!