Add test fixtures to enable CI testing

[Fuabioo]

Problem

Some tests depend on actual Claude Code history existing on the machine, causing them to skip in CI environments.

Evidence

conversation_test.go:9-40

func TestScanConversationMeta(t *testing.T) {
    projectsDir := DefaultProjectsDir()  // ~/.claude/projects
    projects, err := ListProjects(projectsDir)
    if err != nil {
        t.Skipf("No projects found: %v", err)
    }
    if len(projects) == 0 {
        t.Skip("No projects available")
    }
    // ...
}

This test:

• Requires actual Claude Code history in ~/.claude/projects
• Skips in CI (no Claude data)
• Skips on fresh dev machines
• Skips on machines without Claude Code installed

Impact

Tests that skip provide no value in CI - you can't verify the code works correctly in automated pipelines.

Suggested Fix

1. Create test fixtures - Sample JSONL files with realistic conversation data
2. Use a testdata directory - Standard Go convention for test fixtures
3. Mock the filesystem or use temp directories with fixture data
4. Keep existing tests but add parallel fixture-based tests that always run

Example structure:

internal/history/testdata/
  projects/
    -Users-test-project/
      session-abc123.jsonl
      agent-def456.jsonl

Files: internal/history/conversation_test.go:9-40