Add paired specification.json and specification.html generator

Single SPEC source regenerates machine-readable JSON for agents and
dark HTML for humans so architecture survives lost chat sessions.

AGENTS.md

@@ -1,5 +1,20 @@
 # Agent notes (arr-stack)
 
+## Project specification (read first)
+
+| File | Audience |
+|------|----------|
+| **`specification.json`** | Coding agents / tools — structured index of the whole stack |
+| **`specification.html`** | Humans — same content, readable |
+
+**Do not edit those two files by hand.** Update `scripts/generate-specification.py` (`SPEC` dict), then run:
+
+```bash
+./scripts/generate-specification.py
+```
+
+Commit **both** outputs together so they stay in lockstep. Prefer JSON over HTML for agent context.
+
 ## Git
 
 - Remote: `origin` → `ssh://git@192.168.8.123:2222/tim/stack.git` (Forgejo `tim/stack`)