chore(#89,#90,#91): eliminate unwrap, restrict API surface, split pattern.rs

[randomm]

Summary

• Replace all bare unwrap() in library code with expect("descriptive message") (chore: audit and eliminate panicking unwrap() in library code #89)
• Hide CLI internals (commands, help, util) with #[doc(hidden)], add focused pub use re-exports for library consumers (chore: restrict public API surface in lib.rs to library-appropriate items #90)
• Split pattern.rs (500 lines) into pattern/ module directory: mod.rs (273), builtins.rs (138), toml.rs (125) (chore: split pattern.rs to stay under 500-line hard cap #91)

All 197 tests pass. Zero clippy warnings.

Part of EPIC #88 (open-source readiness).

Fixes #89
Fixes #90
Fixes #91