Add agent-to-agent payment landscape brief

[artylobos]

Closes #21

Summary:

• Add a new brief covering x402, MPP, AP2, Circle Nanopayments, and kcolbchain Switchboard.
• Include a comparison table for gas cost, latency, chain support, and maturity.
• Add inline citations and a References section with primary/official sources.
• Link the new brief from the README research brief table.

Verification:

• Word count: 2133 words, within the requested 1500-2500 range.
• Checked Markdown structure against briefs/_TEMPLATE.md sections.
• Ran git diff --cached --check before commit.

[abhicris]

Welcome to kcolbchain, @artylobos — glad you're here. 🌱

Here's what happens from this PR:

1. Our automated review looks for obvious issues (tests, secrets, size) within a couple of hours.
2. If it's clean and CI passes, we merge without back-and-forth.
3. If we need changes, we'll leave a specific comment — not a generic nit. Push another commit and we re-review.

While you wait:

• Run the repo's tests locally (see the repo README.md).
• Keep the PR scoped to one concern — bigger PRs land slower.
• Don't commit tokens or .env contents.

What happens after your first merge

• You're listed on https://kcolbchain.com/contributors/ (opt-out anytime via data/contributors-optout.txt).
• At 2+ merged PRs you're a Regular; at 5+ you're a Fellow — Fellows are the pool we invite first for paid research, partner-org gigs, and grant-funded work. Details: https://kcolbchain.com/docs/org-contribution-policy.html
• Looking for what to pick up next? https://kcolbchain.com/invitations/ lists open help-wanted / good-first-issue across every kcolbchain repo.

Thanks for writing the code. We're building this to last.