chore(governance): land missing docs/packaging before the standards#505 gates bite (#19)

Remediates this repo against the two governance gates promoted from
advisory to blocking in
[hyperpolymath/standards#513](hyperpolymath/standards#513)
(closing standards#505).

Those gates previously **could not fail**: the docs check only emitted
`::warning::`, and the package-policy job ended in an unconditional `✅
Package policy check passed`. They are now real, with a grace window —
they begin **blocking on 2026-08-21**. This repo was measured as
non-compliant, so this PR lands the missing files ahead of that date.

**Added here:** `CONTRIBUTING`

### `CONTRIBUTING.md`
Markdown by estate rule (it is on the GitHub-required community-health
list, unlike `README.adoc`). Content is derived from this repo, not
boilerplate: the build section lists **only `just` recipes that actually
exist here**, and the A2ML section appears only if `.machine_readable/`
is present.

---
Part of an estate-wide sweep; 63 repos were identified as non-compliant.
Third-party/forked repos (`HOL`, `awesome-*`, `lua-filters`) were
**excluded** per the licence policy's do-not-touch rule.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: hyperpolymath

CONTRIBUTING.md

@@ -0,0 +1,61 @@
+<!-- SPDX-License-Identifier: MPL-2.0 -->
+<!-- SPDX-FileCopyrightText: 2026 Jonathan D.A. Jewell -->
+
+# Contributing to julia-ecosystem
+
+Thanks for your interest. This repository follows the Hyperpolymath estate
+standards defined in [hyperpolymath/standards](https://github.com/hyperpolymath/standards).
+
+## Licence
+
+This project is licensed under **MPL-2.0**. By contributing you agree that
+your contributions are licensed under the same terms. Every source file
+carries an `SPDX-License-Identifier` header; keep it when editing, and add
+one to any new file.
+
+## Development environment
+
+Use the toolchain declared by this repository (just).
+
+## Build and test
+
+This repo uses [`just`](https://just.systems) (the estate uses Justfiles,
+never Makefiles). Recipes available here:
+
+```sh
+just            # list recipes
+just doctor     # environment diagnostics
+```
+
+## Machine-readable artefacts
+
+This repo carries `.machine_readable/` A2ML files (`STATE.a2ml`,
+`META.a2ml`, `ECOSYSTEM.a2ml`, `AGENTIC.a2ml`, `NEUROSYM.a2ml`,
+`PLAYBOOK.a2ml`). If your change alters project state, architecture, or
+operational steps, update the corresponding file in the same PR — CI
+validates them.
+
+## Language policy
+
+The estate restricts which languages may be used. In particular Python, Go,
+TypeScript, ReScript, V-lang, Java/Kotlin, Swift and Makefiles are **not**
+accepted in new code; AffineScript, Rust/SPARK, Zig, Deno, Gleam, Elixir,
+Haskell, Idris2, Agda, Julia and OCaml are. CI enforces this, so check the
+policy in `hyperpolymath/standards` before introducing a new language.
+
+## Documentation format
+
+Docs are AsciiDoc (`.adoc`) by default, including `README.adoc`. The
+GitHub-required community-health files stay Markdown: `SECURITY.md`,
+`CONTRIBUTING.md`, `CODE_OF_CONDUCT.md`, `CHANGELOG.md`. Do not add a `.md`
+duplicate of a doc that already exists as `.adoc`.
+
+## Pull requests
+
+1. Branch from `main` — do not push to `main` directly; branch protection
+   requires review and passing checks.
+2. Keep the change focused, and explain *why* in the PR body.
+3. Make sure governance CI is green. It checks documentation presence,
+   packaging policy, secrets, licence consistency and workflow security.
+4. Security issues: follow `SECURITY.md` — report privately, never in a
+   public issue.