feat(cli): add initial simples3 CLI

[rhnvrm]

Summary

• add the first cmd/simples3 CLI with shared AWS config/env resolution and s3:// path parsing
• implement ls, cp, rm, mb, rb, mv, presign, and a first-pass one-way sync
• add command-level tests and README usage docs for the new CLI surface

CLI scope

• shared flags: --profile, --region, --endpoint, --json
• transfer flags: --recursive, --dry-run, --include, --exclude
• sync flag: --delete
• thin wrappers for --acl, --sse, --sse-kms-key-id, --version-id
• sync uses first-pass size + modtime comparison and supports one-way source-of-truth deletes

Notes

• cp and mv require explicit --recursive for S3 prefix-like sources
• command behavior is intentionally smaller than mc; this is the practical first release surface
• supersedes the docs-only roadmap PR in docs: refine CLI roadmap scope #40

Testing

• go test ./cmd/simples3/...
• go build -o /dev/shm/pi-bin/simples3 ./cmd/simples3