refactor(schema): inline the session message ID schema

[opencode-agent]

@kitlangton

Problem

SessionMessage.ID is currently defined indirectly through a standalone session-message-id.ts module:

import { SessionMessageID } from "./session-message-id"

export const ID = SessionMessageID.ID
export type ID = SessionMessageID.ID

The schema package has the real ID module, and Core adds a second two-line wrapper for it. Unlike session-id.ts and workspace-id.ts, this split is not preventing a circular dependency: session-message.ts does not depend on session-event.ts or session-input.ts.

This leaves an extra module and forwarding aliases without a dependency-boundary benefit.

Proposed solution

• Define the branded ID schema and its create() static directly in packages/schema/src/session-message.ts.
• Update schema consumers (session-event.ts and session-input.ts) to use SessionMessage.ID.
• Update Core's session/context-epoch.ts to use SessionMessage.ID.create().
• Delete both wrappers:
  • packages/schema/src/session-message-id.ts
  • packages/core/src/session/message-id.ts

Keep session-id.ts and workspace-id.ts separate: those are shared leaf modules whose inlining would introduce dependency cycles.

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