Add Rust include_dir bundle support for embedded skill trees

[samzong]

Problem

Kitup already treats a skill bundle as a directory tree: SKILL.md, references/, scripts/, assets/, and other regular files should be validated, hashed, and copied together.

The Go SDK has a good embedded-tree path through FSBundle(embed.FS, root), so embedding CLIs can ship a whole skill directory without manually listing every file.

The Rust SDK currently has directory_bundle(path) and files_bundle(files). For a released CLI binary, directory_bundle depends on a runtime filesystem path, while files_bundle pushes Rust CLI authors toward hand-maintaining a SkillFile list with include_bytes!. That becomes brittle as soon as a skill adds references/*.md, scripts, or assets.

Proposal

Add an optional Rust SDK integration for include_dir, exposed as a thin constructor over the existing bundle model.

Possible API shape:

static SKILL: include_dir::Dir<'_> =
    include_dir::include_dir!("$CARGO_MANIFEST_DIR/../skills/mycli");

let bundle = kitup::include_dir_bundle(&SKILL);

Implementation should stay boring:

• add an optional Cargo feature such as include-dir = ["dep:include_dir"];
• traverse include_dir::Dir into Vec<SkillFile>;
• reuse existing files_bundle / normalization / hashing / copy behavior;
• do not introduce a new installer path or new bundle semantics.

Why not build this from scratch?

Rust does not have a standard-library equivalent of Go embed.FS, but the ecosystem already has include_dir, whose explicit purpose is embedding a directory tree into a binary. Kitup should adapt that type instead of becoming a resource-embedding framework.

Acceptance criteria

• Rust SDK can construct a skill bundle from an embedded include_dir::Dir.
• Embedded bundles include nested files such as references/*.md, scripts/*, and assets/* without manually listing every file.
• Existing deterministic hash, skip-file, validation, copy, and ownership behavior is reused.
• Add a focused Rust test or golden coverage proving an embedded directory bundle installs the same tree shape as the current file-list fixture.
• Update Rust README/API docs with the embedded skill bundle example.