feat: scaffold adapter interfaces and module boundaries (issue #16)

[thundermiracle]

Summary

• introduce explicit Rust backend layers: domain, adapters, infra, and application
• define ClientAdapter interface and client profiles/capabilities in the domain layer
• add placeholder adapters for Claude Code, Codex CLI, Cursor, and Codex App
• add AdapterRegistry in infra layer and wire commands through AdapterService
• keep Tauri commands thin and focused on envelope/lifecycle concerns
• add architecture documentation for layer boundaries and extension rules
• add contract tests for module boundaries to prevent layer regressions

Commit Structure

• feat(rust): introduce domain-adapter-infra module boundaries (issue #16)
• test(docs): add module boundary contract checks and architecture notes (issue #16)

Validation

• pnpm run lint
• pnpm test
• cargo test --manifest-path src-tauri/Cargo.toml
• pnpm outdated

Closes #16