Set up :core module with Ktor client and base request/response model

[syncended]

Goal

Introduce a shared :core Gradle module that holds the domain model and HTTP engine used by both :cli and the future TUI module. This is the architectural foundation for #4.

Scope

• New Gradle module :core (Kotlin/JVM, JDK 21).
• :cli depends on :core.
• Add Ktor client to gradle/libs.versions.toml (engine TBD — likely ktor-client-cio or ktor-client-okhttp).
• Add kotlinx.serialization for JSON.
• Domain model:
  • HttpMethod enum (GET/POST/PUT/PATCH/DELETE/HEAD/OPTIONS).
  • Request data class (method, url, query params, headers, body).
  • Body sealed class: Raw(text, contentType), Json(value), FormUrlEncoded(fields), Multipart(parts).
  • Response data class (status, headers, body bytes, elapsed time, content-type).
• RequestExecutor interface + Ktor-backed implementation. Coroutine-based.
• Unit tests for body serialization variants.
• Integration test for executor against a small embedded mock server (Ktor server or MockEngine).

Out of scope

• CLI/TUI surface — handled in Create REST request flow (all HTTP methods, body, headers) — CLI & TUI #4 and TUI main screen #5.
• Persistence / saved requests / history.
• Settings layer.

Acceptance criteria

• :core module builds and tests pass in CI.
• :cli can import and call RequestExecutor to send a real HTTP request.
• No CLI/TUI-specific code lives in :core.