docs(recipes): add a recipes/README explaining the registry snapshot lifecycle

## Summary

`recipes/` has no README. The registry snapshot process lives entirely in a Makefile target, while the issue templates actively route outside contributors into this area with no docs to land on.

## Background

The `recipes/` tree contains only `registry/{0.6.0.json, 0.7.0-beta.1.json, CURRENT}` and no README at any level. The snapshot is produced by `make recipes-registry`, which shells `mlxcel arch --json` and rewrites `CURRENT`. The only prose mention in the repo is a single CHANGELOG bullet, yet `.github/ISSUE_TEMPLATE/recipe_request.yml` sends recipe requests here.

## Proposed Solution

Add a roughly 20-line `recipes/README.md` covering what the registry snapshot is, how `make recipes-registry` regenerates it, what `CURRENT` means, and how the snapshot relates to `mlxcel arch`. All of this is derivable from the Makefile target.

## Implementation Notes

- `recipes/registry/` holds `0.6.0.json`, `0.7.0-beta.1.json`, and `CURRENT`; there is no README (confirmed).
- The generator is `make recipes-registry` (`Makefile:147-160`).
- `CHANGELOG.md:34` is the single existing prose mention.
- `.github/ISSUE_TEMPLATE/recipe_request.yml` routes recipe requests to this area.

## Acceptance Criteria

- [ ] `recipes/README.md` exists and documents the snapshot lifecycle

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## Original Suggestion

### Title: docs(recipes): add a recipes/README explaining the registry snapshot lifecycle

`recipes/` has no README: the registry snapshot process lives entirely in a Makefile target, while the issue templates actively invite outside contributors into this area with no docs to land on.

## Evidence

- `recipes/` contains only `registry/{0.6.0.json, 0.7.0-beta.1.json, CURRENT}` — no README at any level
- The process is `make recipes-registry` (`Makefile:147-160`), which shells `mlxcel arch --json` and rewrites `CURRENT`; the single prose mention in the repo is one CHANGELOG bullet (`CHANGELOG.md:34`)
- `.github/ISSUE_TEMPLATE/recipe_request.yml` routes recipe requests here

## Suggested fix

A ~20-line `recipes/README.md`: what the registry snapshot is, how `make recipes-registry` regenerates it, what `CURRENT` means, and how the snapshot relates to `mlxcel arch`. All derivable from the Makefile target.

## Acceptance criteria

- [ ] `recipes/README.md` exists and documents the snapshot lifecycle

Start with Makefile:147-160 and inspect the existing files under recipes/registry. Confirm how make recipes-registry invokes mlxcel arch --json and rewrites CURRENT, then add recipes/README.md covering the snapshot lifecycle, CURRENT, and the relationship to mlxcel arch. Done means the README exists and documents each requested point.

Written by the indexing model from the issue text.