docs: Add DigitalOcean VPS guide with cost comparison

[0xJonHoldsCrypto]

Summary

Adds a comprehensive guide for running Clawdbot on DigitalOcean, the most beginner-friendly VPS option.

What's included

• Cost comparison table — Oracle (free), Hetzner ($4), DigitalOcean ($6), Vultr ($6), Linode ($5)
• Step-by-step setup — Droplet creation through channel connection
• 1GB RAM optimizations — Swap setup, memory tips
• Dashboard access — SSH tunnel and Tailscale options
• Oracle Cloud Free Tier section — Documents the actually-free option (4 OCPU / 24GB ARM)

Why this matters

• Many users ask in Discord about the cheapest way to run Clawdbot
• The existing Hetzner guide is great but some users prefer DigitalOcean's UI
• Cost comparison helps users make informed decisions
• Oracle Free Tier is often overlooked

Checklist

• Follows existing docs structure (matches hetzner.md format)
• Includes frontmatter with summary/read_when
• Tested commands work on Ubuntu 24.04
• Links to related guides (Hetzner, Docker, Tailscale)

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Contributed via Clawdbot by @0xJonHoldsCrypto 🦖

[thewilloftheshadow]

LGTM.

[thewilloftheshadow]

Merged via rebase. Landed commit: 10914d6.
Follow-up clawtributors update: e040f63.