/commit-msg: Conventional Commits message from staged changes

`/commit-msg` turns the staged diff into a commit message that follows the Conventional Commits format (`type(scope): subject`) and matches how your repository already writes commits.

**What it does** - Injects `git diff --cached` and the last 15 commit subjects into the prompt, so Claude sees exactly what is staged and your house style. - Chooses the type (`feat`, `fix`, `refactor`, `perf`, `docs`, `test`, `build`, `ci`, `chore`, `revert`) based on what the change does, not on which files changed. - Picks a scope from the area of code if your history uses scopes. - Writes a subject in the imperative mood, 72 characters or fewer, without a trailing period. - Adds a body when the reason is not obvious from the subject, and a `BREAKING CHANGE:` footer when a public interface changed. - Warns if the staged changes look like two unrelated changes that should be separate commits.

**What it will not do** It does not stage files, and it does not commit until you confirm. It does not invent issue numbers; it only adds a reference such as `Refs: #123` if you pass one or it appears in the branch name.

**Example** Staged: a fix to rounding in `src/cart/totals.ts` plus a new test. Output: ``` fix(cart): round line totals before summing

Totals were summed at full precision and rounded once, so carts with many discounted items could be off by one cent from the invoice. ```