refactor(machine-readable): rename 6a2/ → descriptiles/ (#35)

## What

Starts the estate move from the `6a2/` naming to **`descriptiles/`**
(parallel to the existing `contractiles/`). casket-ssg is the pilot.

- `git mv .machine_readable/6a2/ → .machine_readable/descriptiles/`
(history preserved) — the six descriptive A2ML files: STATE, META,
ECOSYSTEM, AGENTIC, NEUROSYM, PLAYBOOK.
- Updated the four literal `6a2` references:
- `contractiles/Mustfile.a2ml` — `descriptiles-present` + `anchors`
checks now point at `descriptiles/`
  - `GOVERNANCE.adoc` — Machine-Readable convention row
  - `descriptiles/PLAYBOOK.a2ml` — layout comment
- `.claude/CLAUDE.md` — artefacts list now points at
`descriptiles/*.a2ml` (A2ML), replacing the stale `.scm` list

`a2ml` remains the target format; **Guix/environment config is
untouched** per the standing carve-out.

## Not in this PR (flagged follow-ons)

- **Estate-canonical definition still says `6a2/`** — `standards`
(`meta-a2ml/.claude/CLAUDE.md`, `QUICKSTART-DEV.adoc`,
`ai-instruction/*`, `0-AI-MANIFEST.a2ml`) and estate tooling (`hypatia`,
`gitbot-fleet`, `coordination.k9`) read `6a2/`. casket is intentionally
piloting ahead; the canonical flip + tooling update should follow.
- **The Gnosis loader (`src/Gnosis/SixSCM.hs`) still reads
`.machine_readable/STATE.scm`** (the old flat `.scm` layout) — it does
not yet read `descriptiles/*.a2ml`. Wiring it (needs an A2ML reader) is
the next slice that makes the self-rendering demo actually work.

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

https://claude.ai/code/session_01WN6nPWWe4dvXfoqBG6auWN

---
_Generated by [Claude
Code](https://claude.ai/code/session_01WN6nPWWe4dvXfoqBG6auWN)_

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

Author: hyperpolymath

.claude/CLAUDE.md

@@ -4,14 +4,15 @@ Copyright (c) Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>
 -->
 ## Machine-Readable Artefacts
 
-The following files in `.machine_readable/` contain structured project metadata:
-
-- `STATE.scm` - Current project state and progress
-- `META.scm` - Architecture decisions and development practices
-- `ECOSYSTEM.scm` - Position in the ecosystem and related projects
-- `AGENTIC.scm` - AI agent interaction patterns
-- `NEUROSYM.scm` - Neurosymbolic integration config
-- `PLAYBOOK.scm` - Operational runbook
+The following files in `.machine_readable/descriptiles/` contain structured
+project metadata (A2ML — TOML-like format; formerly the `6a2/` set of `.scm` files):
+
+- `STATE.a2ml` - Current project state and progress
+- `META.a2ml` - Architecture decisions and development practices
+- `ECOSYSTEM.a2ml` - Position in the ecosystem and related projects
+- `AGENTIC.a2ml` - AI agent interaction patterns
+- `NEUROSYM.a2ml` - Neurosymbolic integration config
+- `PLAYBOOK.a2ml` - Operational runbook
 
 ---
 

.machine_readable/contractiles/Mustfile.a2ml

@@ -61,14 +61,14 @@ hooks fail if any check fails.
 - run: test -f .machine_readable/contractiles/bust/Bustfile.a2ml && test -f .machine_readable/contractiles/dust/Dustfile.a2ml
 - severity: critical
 
-### six-directory-present
-- description: 6a2 directory exists with required files
-- run: test -d .machine_readable/6a2 && test -f .machine_readable/6a2/META.a2ml && test -f .machine_readable/6a2/ECOSYSTEM.a2ml && test -f .machine_readable/6a2/STATE.a2ml && test -f .machine_readable/6a2/PLAYBOOK.a2ml && test -f .machine_readable/6a2/AGENTIC.a2ml && test -f .machine_readable/6a2/NEUROSYM.a2ml
+### descriptiles-present
+- description: descriptiles directory exists with the six required A2ML files
+- run: test -d .machine_readable/descriptiles && test -f .machine_readable/descriptiles/META.a2ml && test -f .machine_readable/descriptiles/ECOSYSTEM.a2ml && test -f .machine_readable/descriptiles/STATE.a2ml && test -f .machine_readable/descriptiles/PLAYBOOK.a2ml && test -f .machine_readable/descriptiles/AGENTIC.a2ml && test -f .machine_readable/descriptiles/NEUROSYM.a2ml
 - severity: critical
 
 ### anchors-directory
-- description: anchors directory exists in 6a2
-- run: test -d .machine_readable/6a2/anchors
+- description: anchors directory exists in descriptiles
+- run: test -d .machine_readable/descriptiles/anchors
 - severity: warning
 
 ### self-validating-structure

.machine_readable/6a2/AGENTIC.a2ml …chine_readable/descriptiles/AGENTIC.a2ml.machine_readable/6a2/AGENTIC.a2ml renamed to .machine_readable/descriptiles/AGENTIC.a2ml .machine_readable/6a2/AGENTIC.a2ml renamed to .machine_readable/descriptiles/AGENTIC.a2ml

@@ -61,7 +61,7 @@ enforcement-workflow = ".github/workflows/estate-rules.yml"
 
 # === Required directories ===
 # .github/             CONTRIBUTING.md, CODE_OF_CONDUCT.md, SECURITY.md, workflows/
-# .machine_readable/   AI manifests (0.1-AI-MANIFEST.a2ml), 6a2/ checkpoints,
+# .machine_readable/   AI manifests (0.1-AI-MANIFEST.a2ml), descriptiles/ checkpoints,
 #                      contractiles/, configs/, anchors/, policies/, scripts/, svc/
 # build/               contractile.just, flake.nix, guix.scm, Containerfile,
 #                      just/*.just (Justfile section imports)

.machine_readable/6a2/ECOSYSTEM.a2ml …ine_readable/descriptiles/ECOSYSTEM.a2ml.machine_readable/6a2/ECOSYSTEM.a2ml renamed to .machine_readable/descriptiles/ECOSYSTEM.a2ml .machine_readable/6a2/ECOSYSTEM.a2ml renamed to .machine_readable/descriptiles/ECOSYSTEM.a2ml

@@ -128,7 +128,7 @@ This repository adheres to hyperpolymath estate-wide policies:
 
 | **Contractiles** | Mustfile, Trustfile, Intendfile, Adjustfile in root
 
-| **Machine Readable** | META.a2ml in .machine_readable/6a2/
+| **Machine Readable** | META.a2ml in .machine_readable/descriptiles/
 
 | **CI/CD** | GitHub Actions workflows in .github/workflows/