docs: refresh README first fold, reconcile stale benchmark numbers, unify the one-liner

README first fold:
- banner 800px -> 520px
- badges condensed from two paragraphs into one row (Trendshift kept, inline)
- lead with Introduction instead of benchmarks
- new How it works section explaining the add/search loop
- benchmarks moved below the intro and retitled from the date-stamped
  New Memory Algorithm (April 2026)
- dropped Research Highlights, which restated the benchmark table verbatim

Benchmarks: the README was already correct at 92.5 / 94.4, matching
mem0.ai/research and docs/core-concepts/memory-evaluation.mdx. The stale
copies were in the migration guides, which quote the numbers as a live
reason to upgrade. Updated both to 92.5 / 94.4 (+21 / +27).
docs/changelog/highlights.mdx keeps 91.6 / 93.4 inside its dated
2026-04-14 entry, which records what was announced at the time.

One-liner: standardized on 'the memory layer for AI agents', already the
canonical form in cli-spec.json, cli/python/pyproject.toml and the CLI
specification. Swept the remaining variants.

mem0-ts/package.json

@@ -1,7 +1,7 @@
 {
   "name": "mem0ai",
   "version": "3.1.6",
-  "description": "The Memory Layer For Your AI Apps",
+  "description": "The memory layer for AI agents",
   "main": "./dist/index.js",
   "module": "./dist/index.mjs",
   "types": "./dist/index.d.ts",