fix: respect OPENCLAW_HOME for isolated gateway instances

When OPENCLAW_HOME is set (indicating an isolated instance), the gateway
port should be read from config rather than inheriting OPENCLAW_GATEWAY_PORT
from a parent process. This fixes running multiple OpenClaw instances
where a child process would incorrectly use the parent's port.

Changes:
- resolveGatewayPort() now prioritizes config.gateway.port when OPENCLAW_HOME is set
- Added getConfigPath() function for runtime-evaluated config path
- Deprecated CONFIG_PATH constant with warning about module-load-time evaluation
- Updated gateway run command to use getConfigPath() instead of CONFIG_PATH

Fixes the issue where spawning a sandbox OpenClaw instance from within
another OpenClaw process would fail because OPENCLAW_GATEWAY_PORT from
the parent (set in server.impl.ts) would override the child's config.

src/cli/gateway-cli/run.ts

@@ -4,7 +4,7 @@ import path from "node:path";
 import type { GatewayAuthMode } from "../../config/config.js";
 import type { GatewayWsLogStyle } from "../../gateway/ws-logging.js";
 import {
-  CONFIG_PATH,
+  getConfigPath,
   loadConfig,
   readConfigFileSnapshot,
   resolveStateDir,
@@ -161,7 +161,7 @@ async function runGatewayCommand(opts: GatewayRunOpts) {
   const tokenRaw = toOptionString(opts.token);
 
   const snapshot = await readConfigFileSnapshot().catch(() => null);
-  const configExists = snapshot?.exists ?? fs.existsSync(CONFIG_PATH);
+  const configExists = snapshot?.exists ?? fs.existsSync(getConfigPath());
   const configAuditPath = path.join(resolveStateDir(process.env), "logs", "config-audit.jsonl");
   const mode = cfg.gateway?.mode;
   if (!opts.allowUnconfigured && mode !== "local") {