refactor: centralize main agent id constant

[CoutinhoTTS]

Summary

Centralize the main agent id (main) so runtime code does not duplicate the literal across agent-core, node-sdk, and apps/kimi-code.

Motivation

The main agent id is currently treated as a shared protocol/identity value, but it is defined or hard-coded in multiple places. For example:

• packages/node-sdk/src/rpc.ts defines a local MAIN_AGENT_ID
• packages/node-sdk/src/session.ts defines a local MAIN_AGENT_ID
• packages/agent-core/src/session/rpc.ts checks agentId === 'main'
• apps/kimi-code has its own TUI-level MAIN_AGENT_ID

This makes future changes error-prone because the value is part of the SDK/core communication contract.

Proposed Change

Define MAIN_AGENT_ID once in agent-core and reuse it across the runtime code:

• Export MAIN_AGENT_ID from packages/agent-core/src/agent/index.ts
• Use it in agent-core session/rpc code instead of hard-coded 'main'
• Import it in node-sdk from @moonshot-ai/agent-core
• Re-export it from @moonshot-ai/kimi-code-sdk
• Let apps/kimi-code consume it through the SDK boundary instead of importing agent-core directly

Scope

This is an internal refactor. It should not change runtime behavior.

Validation

Expected validation:

• pnpm --filter @moonshot-ai/agent-core typecheck
• pnpm --filter @moonshot-ai/kimi-code-sdk typecheck
• pnpm --filter @moonshot-ai/kimi-code typecheck

If this direction sounds reasonable, I’d be happy to submit a PR to fix this.