Cards: optional repo link, block-level descriptions, schema + CI validation

- Project card: optional `repo` field renders a git icon corner link
  (github.com / bitbucket.org / Gitea URLs supported).
- All cards: `.card__title` and `.card__desc` are now display:block, so
  the description always starts on its own line.
- Added data/links.schema.json (JSON Schema 2020-12) describing the
  full content model. Both data files reference it via `$schema` for
  github.dev / VS Code autocomplete.
- New GitHub Action validates data/links.json on every push/PR:
  jq syntax check + ajv-cli schema validation.

data/links.example.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "./links.schema.json",
   "profile": {
     "name": "Ada Lovelace",
     "handle": "@ada",
@@ -39,6 +40,7 @@
           "type": "card",
           "title": "Bernoulli Numbers",
           "url": "https://example.com/projects/bernoulli",
+          "repo": "https://github.com/example/bernoulli",
           "description": "Method for computing Bernoulli numbers using the Engine.",
           "tags": ["mathematics"],
           "date": "1843-07"