fix(update.sh): match reinstall.sh's OKX-first install flow

[KillerQueen-Z]

Why

scripts/update.sh — the script users actually hit when they run the live curl https://blockrun.ai/XClawRouter-update | bash one-liner — was missing the OKX-first bootstrap that scripts/reinstall.sh already had.

End result: users running the official one-liner installed the new plugin (v0.12.182) but saw:

• no Bootstrapping OKX Agentic Wallet section (no onchainos auto-install)
• no Next step — log in to your OKX Agentic Wallet block (so they didn't know to run xclawrouter setup)
• a misleading New wallet will be generated on next gateway start message — the plugin no longer auto-generates one, so users refreshed the gateway and silently ended up with no wallet at all

What

Bring update.sh in sync with reinstall.sh:

• Add the same → Bootstrapping OKX Agentic Wallet (onchainos)... preflight that auto-installs the binary via OKX's install.sh, with the existing escape hatches (XCLAWROUTER_SKIP_OKX_INSTALL=1, existing wallet.key, XCLAWROUTER_USE_LOCAL_WALLET=1).
• Replace the misleading post-install fallthrough with one that explains the user will use the OKX wallet (and to look at the Next step block).
• Add the same Next step — log in to your OKX Agentic Wallet: npx @blockrun/xclawrouter setup block at the end, gated on the same "not already set up" condition.

Not in scope

• scripts/update.ps1 (PowerShell sibling) needs the same treatment — tracked separately.

Test plan

• bash -n scripts/update.sh — syntax clean
• Reproduced the missing-bootstrap bug by running the live curl … | bash URL: install completed but no OKX guidance shown, gateway restarted with no wallet, plugin silently failed
• Ran the local fixed scripts/update.sh end-to-end: OKX bootstrap section appears, no misleading "wallet will be generated" line, Next step block prints at the end