docs: add ARCHITECTURE.md documenting mento-core contract subsystems (MEN-42)

[mento-val]

Summary

• Adds ARCHITECTURE.md to the repo root documenting the full mento-core architecture
• Covers all required sections from MEN-42: system overview, 6 contract subsystems, key interaction flows, upgradeability patterns, and entry points
• Written for new contributors and security auditors

Sections covered

1. System overview — what mento-core does
2. Contract subsystems — Broker/Exchange Providers, Oracle/Relayers, Tokens, Governance, Reserve, Liquidity Strategies
3. Key interaction flows — swap flow, oracle update flow, governance proposal flow (with ASCII diagrams)
4. Upgradeability notes — Celo Proxy vs OpenZeppelin Transparent Proxy vs non-upgradeable, with specific contract lists
5. Entry points — table mapping goals to starting contracts

Test plan

• No code changes, documentation only
• File created at repo root: ARCHITECTURE.md

Closes MEN-42

🤖 Generated with Claude Code

[mento-val]

Closing this PR as part of a workflow change. Going forward, all PRs must be submitted through the fork-based review process: branches are pushed to mento-val's personal forks, reviewed by the Engineering Manager, and then the EM opens the upstream PR. This work is valid and will be re-submitted through the proper workflow once Val's fork remotes are configured. See Paperclip issue MEN-61 for context.