refactor(proto): rename UpdateSettings to UpdateConfig for consistency with read path

[johntmyers]

Summary

Renames the UpdateSettings RPC to UpdateConfig to match the read-side naming (GetSandboxConfig, GetGatewayConfig). The RPC handles both settings and policy mutations, so UpdateConfig is more accurate.

Related Issue

Follow-up to #474 (PR review feedback)

Changes

• Proto: UpdateSettings → UpdateConfig, UpdateSettingsRequest/Response → UpdateConfigRequest/Response
• Server: handler method update_settings → update_config
• CLI, TUI, sandbox, e2e (Rust + Python): all call sites and mock trait impls updated

Mechanical rename across 17 files, no behavioral changes.

Testing

• cargo build all affected crates
• cargo test -p openshell-server -p openshell-core all passing
• E2E tests (running in CI)

Checklist

• Follows Conventional Commits
• No behavioral changes