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>

build-scripts/explode-meta.js

@@ -0,0 +1,55 @@
+#!/usr/bin/env node
+
+/**
+ * One-shot migration: splits the root meta.json into per-template files at
+ * blueprints/<id>/meta.json. Kept in the repo for reference and for forks
+ * that need to migrate.
+ *
+ * Usage: node build-scripts/explode-meta.js [path/to/meta.json]
+ */
+
+const fs = require("fs");
+const path = require("path");
+
+const metaPath = process.argv[2] || "meta.json";
+const blueprintsDir = "blueprints";
+
+if (!fs.existsSync(metaPath)) {
+  console.error(`❌ ${metaPath} not found`);
+  process.exit(1);
+}
+
+const entries = JSON.parse(fs.readFileSync(metaPath, "utf8"));
+if (!Array.isArray(entries)) {
+  console.error("❌ meta.json must be an array");
+  process.exit(1);
+}
+
+let written = 0;
+const missingDirs = [];
+
+for (const entry of entries) {
+  if (!entry.id) {
+    console.error(`❌ Entry without id: ${JSON.stringify(entry).slice(0, 80)}`);
+    process.exit(1);
+  }
+  const dir = path.join(blueprintsDir, entry.id);
+  if (!fs.existsSync(dir) || !fs.statSync(dir).isDirectory()) {
+    missingDirs.push(entry.id);
+    continue;
+  }
+  fs.writeFileSync(
+    path.join(dir, "meta.json"),
+    JSON.stringify(entry, null, 2) + "\n"
+  );
+  written++;
+}
+
+console.log(`✅ Wrote ${written} blueprints/<id>/meta.json files`);
+if (missingDirs.length) {
+  console.warn(
+    `⚠️  ${missingDirs.length} entries have no blueprint folder and were NOT written:`
+  );
+  for (const id of missingDirs) console.warn(`   - ${id}`);
+  process.exit(2);
+}