feat(pm): expose embeddable command library

[elrrrrrrr]

Summary

• add an utoo-pm library target and reduce the native binary to a thin wrapper
• promote the command-handler layer itself to the public commands::* API; CLI and embedders call the same functions with no mirrored facade
• give every native CLI capability its own command module, including completions, execute, init, pack, and uninstall
• expose shared initialization, command types, version, and clap metadata
• add an external-crate API test covering every CLI business command

Architecture

CLI adapter ─┐
             ├──> commands::<capability> ──> private service/helper layers
embedder ────┘

Command-line parsing, process exit handling, logging, and presentation remain in the CLI adapter. Process-wide configuration keeps its current first-write-wins semantics.

Validation

• cargo check -p utoo-pm --all-targets --locked
• cargo test -p utoo-pm --lib --locked — 371 passed, 3 ignored
• cargo test -p utoo-pm --test library_api --locked
• cargo clippy -p utoo-pm --all-targets --locked -- --deny warnings
• RUSTDOCFLAGS="-D warnings" cargo doc -p utoo-pm --no-deps --locked
• cargo run -p utoo-pm --bin utoo --locked -- --version
• cargo run -p utoo-pm --bin utoo --locked -- --help