feat(cat,cp): implement cat and cp commands

[jmgilman]

Summary

• Implement blob cat command for printing file contents to stdout
• Implement blob cp command for copying files/directories from archives to local filesystem
• Both commands use HTTP range requests for efficient partial downloads

Features

blob cat <ref> <file>...

• Print file contents to stdout (binary-safe)
• Multiple files concatenated in order
• Validates files exist and rejects directories before output
• Normalizes leading slashes for consistent path handling

blob cp <ref>:<path>... <dest>

• Copy files/directories from archive to local filesystem
• Detects source type (file vs directory) from archive using Stat()
• Supports multiple sources from different refs
• Flags:
  • --recursive/-r (default: true) - copy directories recursively
  • --preserve - preserve file permissions and timestamps
  • --force/-f - overwrite existing files

Implementation Details

• Lazy archive loading: only manifest + index downloaded upfront
• File data fetched via HTTP range requests on demand
• Source type detection uses archive.Stat() (not just trailing slash)
• Consistent overwrite behavior: skip existing by default
• Check destination exists before reading file content (avoids unnecessary downloads)

Test plan

• Unit tests for argument parsing
• Unit tests for destination validation
• Unit tests for copy mode detection
• Linting passes
• All tests pass

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