docs: add SUBMISSION.md narrative for ETHGlobal OpenAgents

[devin-ai-integration]

Summary

Adds SUBMISSION.md — the polished hackathon-submission narrative for ETHGlobal OpenAgents covering:

• Short pitch and demo flow
• The five concrete pain points KeeperKit solves (no first-party agent SDK, drifting tool catalogues, opaque HTTP 402 handling, paywalled local dev, OpenAI lock-in)
• What makes it unique (catalogue-tracking tool surface, x402 as first-class agent concern, single ToolSpec across three frameworks)
• Tech-stack table and prize-fit (KeeperHub Best Use + $250 Builder Feedback bounty)
• Roadmap (x402 settlement helper, webhook-driven catalogue refresh, init templates)

This is a pure documentation change — no source files touched, no behaviour changes, no CI impact.

Review & Testing Checklist for Human

• Skim SUBMISSION.md for any factual claim that has slipped (e.g. tool counts, providers list, demo URL) — these reflect today's live deploy state and should match what judges see at http://178.104.45.97:8420/.
• Confirm you're happy with the marketing tone before pasting into the ETHGlobal submission form. Tweak freely; this is a starting point.

Notes

• Companion to FEEDBACK.md (the $250 Builder Feedback Bounty submission).
• Safe to merge any time; no deploy needed.

Link to Devin session: https://app.devin.ai/sessions/4c74607caacb488d803b3a14a74ee6df
Requested by: @danilaverbena

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