test(ci): add Windows and Linux CI matrix for cross-platform verification

[CodeMonkeyCybersecurity]

Problem

The chat-archive cross-platform refactor uses filepath.ToSlash() and runtime.GOOS for platform awareness, but there is no CI job that actually runs tests on Windows or Linux.

Proposed Fix

Add a GitHub Actions CI matrix with:

• macos-latest (current)
• ubuntu-latest
• windows-latest

Run go test -race ./pkg/chatarchive/... on all three platforms.

Priority

P2 — important for confidence in cross-platform correctness.

Evidence

Current tests include platform-conditional test cases (e.g., runtime.GOOS == "windows" in discover_test.go) but these branches only execute when actually running on that OS.