Architecture decisions: files, diagram references, views and panels

- *.archify-decision files: id (unique, words), title, status, date,
  elements, diagrams, supersedes, and prose; parsed, merged and link-checked
  in the model index; diagnostics on the file
- diagrams point at decisions with decisions: [...] on the diagram, nodes
  and edges; stripped before Archify, unknown ids reported, completion,
  hover and go-to-definition for the ids and for elements/diagrams/
  supersedes in decision files
- Decisions panel on the main.archify page; Decisions rail panel under a
  diagram (hover lights up the node or connection); decisions on element
  cards and in the element dialog; decision view for the file, with a
  lost-model note outside a model
- Archify | New Decision… command and template; editor schemas; the three
  New Model examples carry decisions; docs, changelog, 0.8.0

schemas/archify-decision.schema.json

@@ -0,0 +1,27 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "Archify architecture decision",
+  "description": "One architecture decision of a model. Diagrams point at it with `decisions: [<id>]`.",
+  "type": "object",
+  "additionalProperties": false,
+  "required": [
+    "decision"
+  ],
+  "properties": {
+    "schema_version": {
+      "const": 1
+    },
+    "decision": {
+      "type": "string",
+      "description": "What was decided."
+    },
+    "context": {
+      "type": "string",
+      "description": "The situation and what forced a decision."
+    },
+    "consequences": {
+      "type": "string",
+      "description": "What becomes easier, harder, or must change."
+    }
+  }
+}