Refactor update logic into modular files

[ryanfowler]

Summary

• Split the monolithic update implementation into client, archive, install, lock, and schedule modules
• Keep src/update/mod.rs as the orchestration layer for self-update flow
• Preserve and reorganize update-related tests around the new module boundaries
• Update AGENTS.md to reflect the new layout

Testing

• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1