Move the agent client to the Go SDK

Follow-up to #109 (`internal/agent/client.go`): the agent client should live in the [Go SDK](https://github.com/conductor-oss/go-sdk), not here.

## Why

- **Every other SDK already owns this.** Java has `AgentClient`/`OrkesAgentClient` plus a typed `AgentRequest`; Python has `agent_client.py`/`orkes_agent_client.py`.
- **The CLI is already an SDK consumer.** It depends on `github.com/conductor-sdk/conductor-go` and `internal/agent` is the only resource area with a bespoke client on `internal/transport`.
- **Avoid duplication.** The wire DTOs are private to the CLI. Nothing cross-checks them against the server's `AgentStartRequest`. `Compile` posted a bare config while `Run` wrapped it, and the mismatch went unnoticed.
- **Nobody else can use it.** Under `internal/`, the agent API is unreachable outside CLI.

## Scope

Move the client and models to the SDK; keep the CLI's command and UX layer here.

- Move: the REST client, the DTOs, the agent/execution models.
- Keep: SSE stream rendering, interactive prompts, and `service.go` orchestration (`detectFramework`, register-then-run) — these are CLI concerns.
- Then replace `internal/agent`'s client with the SDK's and bump the pinned SDK version.

No user-facing behaviour change intended.