feat: expose arka primitives over MCP

[Genmin]

Summary

• add arka::mcp with an object-safe ArkaTool trait and a minimal stdio JSON-RPC server for initialize, tools/list, and tools/call
• add MCP tools for native balance reads, swap quote/build metadata, and AgentAccount deposit/balance operations
• add the arka-mcp-server binary and README wiring for Claude Desktop
• cover the server with no-network unit tests plus a stdio integration test that spawns the binary and drives MCP requests

Addresses #11.

Validation

• cargo fmt --check
• cargo build --bin arka-mcp-server
• cargo test --lib
• cargo test --test mcp_stdio
• cargo test --all-targets
• git diff --check

[abhicris]

Welcome to kcolbchain, @Genmin — 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.

[abhicris]

🤖 Audit verdict: safe

Clean MCP server implementation with proper input validation, no credential leakage, no malicious patterns, and comprehensive test coverage; agent deposit operations use InMemoryAgentAccount (testing-only).

Audited by the kcolbchain PR pipeline. See pipeline docs.