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

templates/models/c4-standard-web-app/decisions/external-identity-provider.archify-decision

@@ -0,0 +1,17 @@
+schema_version: 1
+decision:
+  id: external-identity-provider
+  title: Sign in through an identity provider, never store passwords
+  status: accepted
+  date: 2026-09-10
+  elements: [idp, api, spa]
+  diagrams: [0200-containers/web-app.archify]
+context: >
+  The first version kept password hashes in the database and
+  had to handle resets, lockouts and two-factor codes itself.
+decision: >
+  Users sign in through an OpenID Connect identity provider.
+  The single-page app gets a token; the API only verifies it.
+consequences: >
+  No password tables, no reset flows. The identity provider
+  is a hard dependency: when it is down, nobody can sign in.