feat: per-template metadata — eliminate meta.json merge conflicts

- move each template's metadata to blueprints/<id>/meta.json (442 files, byte-identical roundtrip with the old root meta.json)
- generate the served meta.json at build time into app/public/meta.json (gitignored); pnpm dev/build run the generator first
- CI validates every blueprints/<id>/meta.json via generate-meta.js --check (required fields, id/folder match, logo exists, folder<->meta bidirectionality) and rejects any committed root meta.json
- remove root meta.json, dedupe-and-sort-meta.js and build-scripts/process-meta.js (obsolete)
- update CONTRIBUTING.md, AGENTS.md and README.md

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

blueprints/lowcoder/meta.json

@@ -0,0 +1,17 @@
+{
+  "id": "lowcoder",
+  "name": "Lowcoder",
+  "version": "2.6.4",
+  "description": "Rapid business App Builder for Everyone",
+  "logo": "lowcoder.png",
+  "links": {
+    "github": "https://github.com/lowcoder-org/lowcoder",
+    "website": "https://www.lowcoder.cloud/",
+    "docs": "https://docs.lowcoder.cloud/lowcoder-documentation"
+  },
+  "tags": [
+    "low-code",
+    "no-code",
+    "development"
+  ]
+}