feat: content cat alias for content show --raw

[flyingrobots]

Motivation

git mind content show --raw is the most common content retrieval pattern (pipe content to other tools). A shorter alias like git mind content cat would match git UX conventions (git cat-file).

Proposed Behavior

$ git mind content cat doc:readme
# Hello World
...

$ git mind content cat doc:readme | wc -l
42

Equivalent to git mind content show doc:readme --raw.

Implementation

• Add cat as a content subcommand alias in bin/git-mind.js
• Route to contentShow(cwd, nodeId, { raw: true })