feat: add AWS doctor command - #144

Summary

• Add a dynavec doctor CLI command
• Check AWS credentials and STS identity
• Check S3 Vectors index access
• Check DynamoDB table access
• Provide clear pass/fail output and exit codes
• Add credential-free unit tests for success and failure cases

Usage

dynavec doctor
dynavec doctor --bucket my-vectors --index docs --table dynavec_docs

The command performs read-only checks and does not create AWS resources.

Validation

uv run --no-project pytest -q
uv run --no-project ruff check src tests benchmarks
uv run dynavec --help
uv run dynavec doctor --help

Closes #87