Security: Add validation and fix concurrency issues in Bushel

[leogdion]

Problem

The Bushel example has several security concerns:

1. Missing PEM file validation: Code attempts to use private key files without validating they exist (SyncCommand.swift:85)
2. Unsafe concurrency: Logger.swift:30 uses nonisolated(unsafe) for isVerbose flag
3. Inadequate error handling: No validation of key ID format before auth

Proposed Solution

1. Add file validation before auth
2. Replace nonisolated(unsafe) with @MainActor or atomic operations
3. Validate key ID format and file permissions

Impact

• Security: Prevents auth with missing/invalid credentials
• User Experience: Clear error messages
• Concurrency: Prevents data races

References

• PR #129 Review: update from writing blog post and examples MistKit#129 (comment)
• Section: Security & Privacy
• Related: #136