feat(12-01): add CLI key args and refactor archive functions for user-specified keys

- Add hex dependency for --key hex decoding
- Add KeyArgs (--key, --key-file, --password) as clap arg group on top-level CLI
- Replace hardcoded KEY constant with resolve_key() supporting hex and file sources
- Refactor pack/unpack to require key parameter, inspect accepts optional key
- Wire CLI key resolution to archive functions in main.rs
- Inspect works without key (header only) or with key (full TOC listing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.toml

@@ -13,6 +13,7 @@ clap = { version = "4.5", features = ["derive"] }
 rand = "0.9"
 rayon = "1.11"
 anyhow = "1.0"
+hex = "0.4"
 
 [dev-dependencies]
 tempfile = "3.16"