refactor(core): inline gateway model factory into AgentSessionDO

[mikea]

Motivation

• Reduce indirection by placing the AI Gateway model construction next to the DO that uses it and keep the spec and implementation in sync.
• Make the AgentSessionDO initialization and model-switch paths self-contained so tests can reliably override the model via _setModelForTest() inside the Durable Object.

Description

• Added an internal createModel(env, modelId) helper and ai-gateway-provider imports directly in packages/core/src/agent-session-do.ts and used it for initialization and model-switching.
• Removed the now-redundant packages/core/src/gateway.ts file.
• Updated packages/core/test/do/mock-model.ts comments to reference the inlined helper location.
• Updated specs/core.md and specs/implementation_plan.md to point at the new createModel() location so spec and code remain aligned.

Testing

• Ran TypeScript type-check via pnpm -r exec tsc --noEmit, which completed successfully.

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Codex Task