Extract Command Handlers into a Library Crate

[brdv]

Summary

Extract the runtime entrypoint into src/lib.rs and move command implementations out of src/cli.rs so the CLI definition stays focused on parsing and dispatch.

This keeps the current behavior intact while making the codebase easier to test, document, and evolve before the upcoming Rustdoc and linting pass.

Changelist

• src/lib.rs adds the library entrypoint used by src/main.rs
• src/main.rs now delegates directly to stck::run()
• src/commands.rs holds the command execution logic that used to live in src/cli.rs
• src/cli.rs now focuses on clap types, parsing, and dispatch

Stack parent: #39
Stack child: none yet

Checklist

• Changes are minimal and focused for this milestone
• No breaking CLI changes unless explicitly intended
• Errors are user-facing, actionable, and prefixed with error:
• Added/updated tests for behavior changes
• Updated docs/plan if behavior or scope changed