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

src/webview/messages.ts

@@ -11,6 +11,7 @@ export type WebviewMessage =
   | { type: 'openSource'; path: string; line?: number; endLine?: number; href?: string }
   | { type: 'openDiagram'; path: string }
   | { type: 'openRegistry'; file: string; line?: number }
+  | { type: 'openDecision'; file: string }
   | { type: 'error'; message: string };
 
 /** Column for opening files next to a webview without covering it. */