chore(deps): bump engines to >=22.5 to match node:sqlite usage in keys.mjs (#62)

OCP uses Node.js built-in SQLite (`node:sqlite`) in keys.mjs:3 for the
LAN-mode key store. The `node:sqlite` module is only available in:

- Node 22.5.0+ behind --experimental-sqlite flag
- Node 23.0.0+ without any flag (fully stable)

The previous `engines: ">=18"` was inaccurate and would have caused
opaque "Cannot find module 'node:sqlite'" failures for users on
Node 18-22.4 the moment LAN mode (multi-key) was enabled.

Changes:
- package.json: engines.node ">=18" → ">=22.5"
- README.md: prerequisite "Node.js 18+" → "Node.js 22.5+ (Node 23+ recommended …)"
  with a one-line note explaining the flag distinction so users on 22.x know
  they may need --experimental-sqlite.

Verification:
- Source usage of node:sqlite confirmed via grep: keys.mjs:3
  (`import { DatabaseSync } from "node:sqlite";`).
- Local node --version = v25.8.0 (well above 22.5); `npm install` exit 0
  with no engine-mismatch warning.
- No other source file imports node:sqlite (single point of usage).

Refs: audit side-finding 2 of 4.

Co-authored-by: dtzp555 <dtzp555@gmail.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -83,7 +83,7 @@ OCP has two roles: **Server** (runs the proxy, needs Claude CLI) and **Client**
 > **Recommended:** Install OCP on a device that stays powered on — Mac mini, NAS, Raspberry Pi, or a desktop that doesn't sleep. This ensures all clients always have access.
 
 **Prerequisites:**
-- Node.js 18+
+- Node.js 22.5+ (Node 23+ recommended — `node:sqlite` is fully stable without flags from 23.0; on 22.5–22.x it works behind `--experimental-sqlite`)
 - [Claude CLI](https://docs.anthropic.com/en/docs/claude-cli) installed and authenticated (`claude auth login`)
 
 ```bash