feat(plugin): expose synthetic session input

[kitlangton]

What
Expose ctx.session.synthetic to Promise and Effect V2 plugins so long-lived plugin work can durably deliver synthetic input back into a session.

How

• Add synthetic to both public plugin session domains.
• Forward Effect plugins directly to the existing runtime.session.synthetic operation.
• Adapt Promise inputs across the branded ID and nullable optional-value boundary.
• Document the capability and cover Promise forwarding and null normalization.

Scope
This only exposes the existing session operation. It does not add a new protocol endpoint or change synthetic input scheduling semantics.

Testing

• bun run test test/plugin/promise.test.ts in packages/core
• bun run test in packages/plugin
• bun run typecheck in packages/core
• bun run typecheck in packages/plugin
• Full monorepo typecheck via the pre-push hook