Add error handling/logging for silent failures

[Fuabioo]

Problem

Several places in the codebase silently ignore errors, which can lead to confusing behavior for users when operations fail without any indication.

Evidence

1. internal/cli/list.go:79

projects, _ := history.ListProjects(cfg.ProjectsDir)
projectCount = len(projects)

If ListProjects fails, projectCount becomes 0 with no indication of failure.

2. internal/cli/stats.go:62-72

conversations, err := scanner.ScanAll()
if err == nil {
    for _, c := range conversations {
        // ...
    }
}

Stats command shows partial or empty data if scanning fails, with no warning to the user.

3. internal/history/scanner.go:103-106

projectFiles, err := s.scanDir(projectDir)
if err != nil {
    continue // Skip directories we can't read
}

Permission errors or corrupted directories are silently skipped.

Impact

Users may see incorrect or incomplete data without knowing something went wrong. This makes debugging issues difficult.

Suggested Fix

Consider either:

1. Return errors up the call stack so they can be handled appropriately
2. Add warning logs when errors are encountered but recoverable
3. Include a summary of any skipped items in the output (e.g., "Scanned 10 projects, 2 skipped due to permission errors")