docs(readme): convert README.adoc -> Markdown (renders on Glama/profile/community-health) (#57)

README was AsciiDoc → renders as raw markup in Markdown consumers (Glama
MCP directory, GitHub community-health, GitHub profile). pandoc
asciidoc→GFM (badges → clickable), SPDX header kept, duplicate `.adoc`
removed. Part of the estate README-format fix.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: hyperpolymath

README.adoc

@@ -0,0 +1,190 @@
+<!--
+SPDX-License-Identifier: CC-BY-SA-4.0
+SPDX-FileCopyrightText: 2025-2026 Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
+-->
+
+[![OpenSSF Best Practices](https://img.shields.io/badge/OpenSSF-Best_Practices-green?logo=opensourcesecurity)](https://www.bestpractices.dev/en/projects/new?repo_url=https://github.com/hyperpolymath/casket-ssg)
+[![License: MPL-2.0](https://img.shields.io/badge/License-MPL_2.0-blue.svg)](https://github.com/hyperpolymath/casket-ssg/blob/main/LICENSE)
+![Accessibility](https://img.shields.io/badge/a11y-WCAG--aware_by_default-success)
+[![poly-ssg](https://img.shields.io/badge/part_of-poly--ssg-indigo)](https://github.com/hyperpolymath/poly-ssg)
+[![RSR Compliant](https://img.shields.io/badge/RSR-compliant-gold)](https://github.com/hyperpolymath/rhodium-standard-repositories)
+
+**An accessibility-first, metadata-driven static site generator.**
+Markdown in, an accessible website out — WCAG-aware output *by default*,
+plus a metadata layer (**Gnosis**) that renders pages from typed project
+data. The Haskell member of
+[poly-ssg](https://github.com/hyperpolymath/poly-ssg).
+
+> [!NOTE]
+> **Status: alpha.** The `build` / `clean` pipeline works and generates
+> real sites. Items marked *Planned* below are not yet implemented. The
+> engine’s direction — demote Pandoc, adopt a Djot prose core + `a2ml`
+> typed content on the
+> [boj](https://github.com/hyperpolymath/boj-server) substrate — is
+> recorded in [ADR-0001](docs/decisions/ADR-0001-content-engine.adoc).
+> Architecture & invariants: [EXPLAINME](EXPLAINME.adoc).
+
+<div id="toc">
+
+</div>
+
+# Example
+
+A minimal site is a tree of Markdown with front-matter:
+
+    content/
+    ├── index.md
+    └── posts/
+        └── 2026-06-23-hello.md
+
+```markdown
+---
+title: Hello, world
+date: 2026-06-23
+tags: [intro]
+description: A first page built with casket.
+---
+# Hello
+
+Markdown with **rich** formatting, tables, footnotes and highlighted code.
+```
+
+Build it:
+
+```bash
+casket-ssg build ./content ./_site
+```
+
+You get an **accessible** `_site/` with no configuration:
+
+- `index.html` and `posts/2026-06-23-hello/index.html` rendered through
+  an accessible default theme (skip link, labelled landmarks, light/dark
+  `color-scheme`), with table headers auto-scoped for WCAG 1.3.1;
+
+- a date-sorted collection index for `posts/`, and per-tag pages under
+  `/tags/`;
+
+- `sitemap.xml` and an Atom `feed.xml`.
+
+No Node, no npm, no required config file.
+
+# Installation
+
+casket is not yet on Hackage — build from source (GHC 9.4+, Cabal 3 or
+Stack 2):
+
+```bash
+git clone https://github.com/hyperpolymath/casket-ssg
+cd casket-ssg
+stack build                         # or: cabal build
+stack run casket-ssg -- --version   # or put the built binary on PATH
+```
+
+CLI:
+
+```bash
+casket-ssg build <input-dir> [output-dir]   # default output: _site
+casket-ssg build --drafts <input-dir>       # include draft: true pages (or CASKET_DRAFTS=1)
+casket-ssg build --no-clean-urls <input-dir> # emit foo.html, not foo/index.html
+casket-ssg clean [output-dir]
+casket-ssg --version
+```
+
+# What makes it different
+
+Accessibility by construction  
+The default theme is accessible by default, casket rewrites Pandoc table
+headers to add `scope="col"`, and the Gnosis `FlexiText` type makes
+**empty alt-text a constructor error** — generated badges cannot ship
+without alt text. Accessibility is a property of the engine, not a
+checklist for the author.
+
+Metadata-driven (Gnosis)  
+An optional layer renders pages from a typed metadata context:
+`(:placeholder)` substitution, `{{#if}}` / `{{#for}}` conditionals and
+loops (DAX), and Shields.io badges generated from typed values — sourced
+from the repo’s `.machine_readable/descriptiles/` data.
+
+Verified & agent-native — the trajectory  
+casket is heading toward typed `a2ml` content (build fails on malformed
+data), a formally-verifiable Djot core, and a verified agent surface as
+a [boj](https://github.com/hyperpolymath/boj-server) cartridge rather
+than an embedded MCP server. See
+[ADR-0001](docs/decisions/ADR-0001-content-engine.adoc).
+
+Permissively licensed  
+**MPL-2.0** — build proprietary or differently-licensed sites on top,
+unlike copyleft alternatives.
+
+# Features
+
+**Implemented**
+
+- Recursive multi-page build mirroring the source tree
+
+- Front-matter: `title`, `date`, `description`, `layout`, `draft`,
+  `tags`, `slug`, plus arbitrary keys as `{{key}}` template vars
+
+- Rich Markdown via Pandoc — anchors, pipe tables, fenced/highlighted
+  code, footnotes, task lists, strikethrough, smart punctuation
+
+- File templates with per-page `layout`, partials (`{{>` `name}}`),
+  `{{nav}}`, `{macro}`, `{{site.*}}` (from `site.conf`)
+
+- Collections (date-sorted directory index) and per-tag pages
+  (`/tags/<tag>/`)
+
+- `sitemap.xml` + Atom `feed.xml`
+
+- Accessible default theme + automatic `scope="col"` on table headers
+
+- Gnosis layer — `(:placeholder)`, `{{#if}}`/`{{#for}}` (DAX),
+  `FlexiText` badges
+
+- Clean URLs, toggleable
+
+**Planned** (do not assume these work today — see ADR-0001)
+
+- `init` scaffolding and a `serve` / live-reload dev server
+
+- Incremental rebuilds; asset fingerprinting
+
+- `a2ml`-native typed content + the Djot core; Pandoc relegated to an
+  importer
+
+- `boj` cartridge registration; `llms.txt` / per-page machine views
+
+# Documentation
+
+- [EXPLAINME](EXPLAINME.adoc) — architecture & invariants (developer
+  deep-dive)
+
+- [ADR-0001](docs/decisions/ADR-0001-content-engine.adoc) —
+  content-engine direction
+
+- [Contributing](CONTRIBUTING.adoc) · [Governance](GOVERNANCE.adoc)
+
+# Where it fits
+
+- [**poly-ssg**](https://github.com/hyperpolymath/poly-ssg) — the family
+  of language-native SSGs casket belongs to, unified via MCP.
+
+- [**ddraig**](https://github.com/hyperpolymath/ddraig-ssg) — the
+  dependently-typed (Idris2) sibling; casket’s "proven twin" (casket
+  validates, ddraig proves).
+
+- [**boj-server**](https://github.com/hyperpolymath/boj-server) — the
+  estate’s verified MCP/capability server casket plugs into as a
+  cartridge.
+
+# Requirements
+
+- GHC 9.4+ (CI builds with 9.4.8), Cabal 3.0+ or Stack 2.0+
+
+- Pandoc (current library dependency; see ADR-0001 for the planned move
+  off it)
+
+# License
+
+Code: **MPL-2.0** ([LICENSE](LICENSE)). Prose documentation:
+`CC-BY-SA-4.0`.