docs: add ZXDB guide; refresh README & AGENTS

Expand and update documentation to reflect the current app (Registers + ZXDB Explorer), with clear setup and usage instructions.

Changes
- README: add project overview including ZXDB Explorer; routes tour; ZXDB setup (DB import, helper search tables, readonly role); environment configuration; selected API endpoints; implementation notes (Next 15 async params, Node runtime for mysql2, SSR/ISR usage); links to AGENTS.md and docs/ZXDB.md.
- docs/ZXDB.md (new): deep-dive guide covering database preparation, helper tables, environment, Explorer UI, API reference under /api/zxdb, performance approach (helper tables, parallel queries, ISR), troubleshooting, and roadmap.
- AGENTS.md: refresh Project Overview/Structure with ZXDB routes and server/client boundaries; document Next.js 15 dynamic params async pattern for pages and API routes; note Drizzle+mysql2, Node runtime, and lookup `text`→`name` mapping; keep commit workflow guidance.
- example.env: add reference to docs/ZXDB.md and clarify mysql:// format and setup pointers.

Notes
- Documentation focuses on the current state of the codebase (what the code does), not a log of agent actions.
- Helper SQL at ZXDB/scripts/ZXDB_help_search.sql is required for performant searches.

Signed-off-by: Junie@lucy.xalior.com

src/app/zxdb/ZxdbExplorer.tsx

@@ -121,7 +121,7 @@ export default function ZxdbExplorer({
   }
 
   return (
-    <div className="container">
+    <div>
       <h1 className="mb-3">ZXDB Explorer</h1>
       <form className="row gy-2 gx-2 align-items-center" onSubmit={onSubmit}>
         <div className="col-sm-8 col-md-6 col-lg-4">