docs: update memU Bot section with GitHub reference and "Now open source"

[sairin1202]

📝 Pull Request Summary

Update the memU Bot (OpenClaw alternative) section in the main README to reference the memUBot GitHub repo, add a clear product tagline and "Now open source," and refine the bullet points (one-click install, 24/7, token cost).

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✅ What does this PR do?

• README: Revise the "OpenClaw (Moltbot, Clawdbot) Alternative" block to use memUBot as the primary reference.
• Links: Point the section title and a new "Source" link to the memUBot GitHub repo; keep the "Try now" link to memu.bot.
• Copy: Add a one-line tagline for memU Bot, add "Now open source," and tighten the bullets (one-click install < 3 min, 24/7, ~1/10 token usage).
• Motivation: Align the README with the open-source memUBot repo and make it clear the bot is now open source and easy to try from GitHub and memu.bot.

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🤔 Why is this change needed?

• Readers need a single, clear link to the memUBot source (GitHub) and a way to try it (memu.bot).
• Stating "Now open source" and the new tagline/bullets improves discoverability and sets expectations (enterprise-ready, proactive, cost-efficient) without changing any code or behavior.

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🔍 Type of Change

Please check what applies:

• Bug fix
• New feature
• Documentation update
• Refactor / cleanup
• Other (please explain)

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✅ PR Quality Checklist

• PR title follows the conventional format (e.g. docs: update memU Bot section and add GitHub reference)
• Changes are limited in scope and easy to review (README only)
• Documentation updated where applicable (this is the doc change)
• No breaking changes (or clearly documented)
• Related issues or discussions linked (add if you have one)

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📌 Optional

• Screenshots or examples added (not needed for README text)
• Edge cases considered (links and wording only)
• Follow-up tasks mentioned

[Copilot]

Pull request overview

Updates the README’s memU Bot / OpenClaw alternative section to better point readers to the open-source memUBot repository while keeping a direct “try it” link.

Changes:

• Switch the section header link to the memUBot GitHub repository.
• Add a memU Bot tagline including “Now open source,” and refine the value-prop bullets.
• Add an explicit “Source” link alongside the existing “Try now” link.

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[Copilot]

The tagline uses sentence fragments (e.g., "The enterprise-ready OpenClaw.") which reads awkwardly in the README. Consider rewriting this into a single complete sentence (or two complete sentences) for clarity.

Suggested change

	**[memU Bot](https://github.com/NevaMind-AI/memUBot)** — Now open source. The enterprise-ready OpenClaw. Your proactive AI assistant that remembers everything.
	**[memU Bot](https://github.com/NevaMind-AI/memUBot)** — now open source, is an enterprise-ready alternative to OpenClaw and your proactive AI assistant that remembers everything.

[Copilot]

This bullet has awkward grammar ("Download-and-use and simple") and is the only place in the README using an HTML entity ("<") for plain text. Consider rephrasing (e.g., "Simple to download and use" / "under 3 min") so the Markdown stays consistent and more readable.

Suggested change

	- **Download-and-use and simple** to get started (one-click install, < 3 min).
	- **Simple to download and use** to get started (one-click install, under 3 min).