Add #[deny(missing_docs)] to all crates

[mroczect]

Summary

Enable the #![deny(missing_docs)] attribute in every crate of the workspace so that any public item without documentation causes a compilation error. This ensures all public APIs are documented from the start.

Context

The handler crate already has this attribute, but core, plumbing, and porcelain do not. Without it, undocumented public items may slip through.

Tasks

• Add #![deny(missing_docs)] to the root lib.rs of:
  • libvctrl_core
  • libvctrl_plumbing
  • libvctrl_porcelain
• Resolve all resulting compilation errors by adding missing doc comments.
• Verify that cargo build passes for the whole workspace.
• Optionally, add a CI step that runs cargo doc and fails on warnings.

Acceptance Criteria

• cargo build succeeds for each crate with #![deny(missing_docs)] active.
• cargo doc emits no warnings related to missing documentation.
• CI (if configured) includes this check.

Dependencies

• None.

References

• Rustdoc: The #[doc] attribute