docs(readme): convert README.adoc -> Markdown README.md

README must be real Markdown to render in GitHub community-health, the GitHub
profile, and external MCP directories (Glama) — AsciiDoc shows as raw markup
there. pandoc asciidoc->GFM, badges fixed to clickable, SPDX header kept as an
HTML comment, duplicate README.adoc removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -1,60 +1,64 @@
-[![Sponsor](https://img.shields.io/badge/Sponsor-%E2%9D%A4-pink?logo=github)](https://github.com/sponsors/hyperpolymath)
+<!--
+SPDX-License-Identifier: CC-BY-SA-4.0
+SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+-->
 
-= asdf Tool Plugins
-// SPDX-License-Identifier: CC-BY-SA-4.0
-:author: Jonathan D.A. Jewell <6759885+hyperpolymath@users.noreply.github.com>
-:revdate: 2026-05-26
+Hyperpolymath’s collection of [asdf](https://asdf-vm.com/) plugins for
+installing and managing development tools and runtimes across the
+estate.
 
-Hyperpolymath's collection of https://asdf-vm.com/[asdf] plugins for installing
-and managing development tools and runtimes across the estate.
+Each top-level subdirectory is a standalone asdf plugin. The full set
+covers the languages, runtimes, and tools the hyperpolymath
+language-policy (<a
+href="https://github.com/hyperpolymath/standards/blob/main/.claude/CLAUDE.md"
+class="md
+in `hyperpolymath/standards`">CLAUDE</a>) currently endorses, plus some
+neighbouring tools used in supporting infrastructure.
 
-Each top-level subdirectory is a standalone asdf plugin. The full set covers
-the languages, runtimes, and tools the hyperpolymath language-policy
-(link:https://github.com/hyperpolymath/standards/blob/main/.claude/CLAUDE.md[CLAUDE.md
-in `hyperpolymath/standards`]) currently endorses, plus some neighbouring
-tools used in supporting infrastructure.
+# Installing a plugin
 
-== Installing a plugin
+Each plugin is installable directly from this monorepo using the
+standard asdf plugin-add URL form:
 
-Each plugin is installable directly from this monorepo using the standard
-asdf plugin-add URL form:
-
-[source,bash]
-----
+```bash
 asdf plugin add <name> https://github.com/hyperpolymath/asdf-tool-plugins.git
-----
+```
 
-(Some plugins may require additional setup — see the plugin's own README
+(Some plugins may require additional setup — see the plugin’s own README
 inside its subdirectory.)
 
-== Layout
+# Layout
 
-This is a *consolidated monorepo* — every plugin lives in a sibling
-directory at the top level. Each is autonomous (its own
-`bin/list-all`, `bin/install`, plugin-specific helpers).
+This is a **consolidated monorepo** — every plugin lives in a sibling
+directory at the top level. Each is autonomous (its own `bin/list-all`,
+`bin/install`, plugin-specific helpers).
 
 Browse the plugin list directly by running:
 
-[source,bash]
-----
+```bash
 ls -d */
-----
+```
+
+# Status
 
-== Status
+- **Licence**: MPL-2.0.
 
-* **Licence**: MPL-2.0.
-* **Estate role**: development-tooling support; downstream of the
+- **Estate role**: development-tooling support; downstream of the
   `hyperpolymath/standards` language-policy.
-* **Audit findings**: see `docs/tech-debt-2026-05-26.md` if present
+
+- **Audit findings**: see `docs/tech-debt-2026-05-26.md` if present
   (added by the 2026-05-26 estate tech-debt scan).
 
-== Contributing
+# Contributing
+
+PRs welcome — see <a href="CONTRIBUTING.md" class="md">CONTRIBUTING</a>.
+All commits must be GPG-signed and follow conventional commits (per
+estate-wide policy in `hyperpolymath/standards`).
 
-PRs welcome — see link:CONTRIBUTING.md[CONTRIBUTING.md].
-All commits must be GPG-signed and follow conventional commits
-(per estate-wide policy in `hyperpolymath/standards`).
+# Companion repositories
 
-== Companion repositories
+- [`hyperpolymath/standards`](https://github.com/hyperpolymath/standards)
+  — canonical estate-wide standards including the language policy.
 
-* link:https://github.com/hyperpolymath/standards[`hyperpolymath/standards`] — canonical estate-wide standards including the language policy.
-* link:https://github.com/hyperpolymath/rsr-template-repo[`hyperpolymath/rsr-template-repo`] — the canonical project scaffold (this repo follows the RSR pattern).
+- [`hyperpolymath/rsr-template-repo`](https://github.com/hyperpolymath/rsr-template-repo)
+  — the canonical project scaffold (this repo follows the RSR pattern).