test: add comprehensive integration tests for all CLI commands

Add integration tests for previously uncovered commands:
- cat: display object contents
- head: display first N lines of object
- pipe: stream stdin to object
- find: find objects by name, path, and size
- diff: compare two bucket locations
- mirror: sync local directory to S3
- tree: display objects in tree format
- version: bucket versioning operations
- tag: object tag management (set, ls, rm)
- alias: complete lifecycle (set, ls, info, rm)

This increases test coverage from ~10 commands to all 20 CLI commands.
Tests gracefully skip when features are not supported by the S3 backend.

Author: overtrue